Abstract: The modular high-temperature gas-cooled reactor (HTGR) nuclear power plant is an advanced next-generation nuclear facility independently developed by China with proprietary intellectual property rights. For HTGR, core water ingress accidents represent a unique and potentially severe safety concern. Due to the high carbon content in key components of HTGR, such as fuel element coated particles and reactor internals, excessive moisture in the primary circuit helium poses significant safety risks. Elevated water vapor introduces positive reactivity, leading to increased core power and temperature. Concurrently, chemical reactions between water vapor and graphite structures generate water gas (CO+H₂), causing a rise in primary circuit pressure. This may trigger safety valve activation, resulting in coolant overpressure discharge and potential radioactive release. The primary circuit helium humidity serves as a distinctive process protection parameter differentiating HTGR from other reactor types, requiring real-time monitoring during operation. When humidity levels exceed safety thresholds due to factors like steam generator leakage, the system must reliably trigger alarms or reactor trip signals to prevent serious consequences. In HTGR engineering, the humidity of the primary circuit helium, a reactor protection variable, was sampled through the following process: The sampling gas, driven by the pressure head of the main helium circulator, was extracted from the circulator outlet. It was then cooled by a cooling device to a temperature range suitable for the measurement instruments, entered the humidity sampling chamber for analysis, and finally returned to the main helium circulator inlet. The humidity sensor is a critical instrument that converts the helium humidity in the sampling chamber into a 4-20 mA current signal, which is transmitted to the reactor protection system. Nuclear safety-grade instrumentation has unique characteristics with requirements distinct from conventional industrial instruments. For nuclear-grade instruments containing embedded software in electrical components, mandatory verification and validation (V&V) of the software is required. Currently, the manufacturing of high-performance humidity sensors and their software source codes are predominantly controlled by a few foreign companies. Due to commercial confidentiality concerns, foreign manufacturers are reluctant to provide source codes for nuclear-grade certification and software V&V, making nuclear safety-grade humidity sensors a critical “chokepoint” issue requiring urgent resolution in HTGR scale-up development. To address import dependence on core humidity measurement devices for HTGR primary circuits, in this paper non-standard large-size high-sensitivity domestic capacitive humidity sensing elements, glass-insulated electrical penetrations, and digital signal processing circuits using domestic microcontroller chips and components were developed, and the V&V for embedded software codes was conducted. The humidity-sensitive capacitive element utilizes a ceramic-based substrate material, a polymer-based humidity-sensitive layer, and gold-plated electrodes. Its surface is protected via thick-film process spraying, providing robustness exceeding conventional industrial standards, thereby establishing a foundation for application in high-reliability environments such as nuclear power plants. The humidity sensor adopts an independent capacitive design, where the capacitor is fabricated separately and connected via rigid pins. Through electrical penetration assemblies, the capacitor is isolated within high-pressure zones, while the circuit components remain in normal atmospheric conditions. The electrical penetration assemblies incorporate glass insulators sealed with metal casings and conductors, validated through insulation resistance testing, helium leak detection, and hydrostatic pressure testing to ensure insulation, sealing, and pressure-bearing capabilities. The conversion circuit employs a domestically produced MCU (STC8G1K17), along with other localized components and internal software code, achieving full intellectual property independence. The fully domestic digital humidity sensor prototype successfully passed accuracy testing, electromagnetic compatibility (EMC) testing, environmental compatibility testing, thermal aging tests, vibration aging tests and seismic resistance tests. This study resolved key technical challenges in sensor localization, verified solution feasibility, and established core proprietary technologies and software codes. It lays the foundation for domestic nuclear safety-grade humidity sensor supply, and will play a vital role in future HTGR operations and modular HTGR power plant commercialization. The achievements provide crucial technical support for achieving complete domestic substitution of nuclear-grade humidity measurement equipment.