Abstract: The high-temperature gas-cooled reactor (HTGR) technology in China has gone through stages of tracking, leapfrogging, and independent innovation, and currently holds a world-leading position in commercial-scale modular HTGR nuclear power plant technology. In HTGRs, the medium temperatures of primary and secondary circuits serve as crucial reactor protection parameters. The temperature measurement points inside the reactor pressure vessel, which monitor critical components like the core support structure, also play significant roles in auxiliary judgment of reactor operational status. As HTGR technology further advances toward ultra-high-temperature gas-cooled reactors, the operating temperatures at these measurement points have reached 750 ℃ or higher, posing challenges to the high-temperature resistance and reliability requirements of temperature measurement components. Currently, the mature nuclear-grade temperature sensor is the K-type (NiCr-NiAl) sheathed thermocouple. However, the performance of traditional stainless steel-clad K-type sheathed thermocouples is approaching material limits, as their cladding strength and oxidation resistance significantly decrease under higher temperatures, thereby affecting long-term reliability. The N-type thermocouple improves upon the K-type by optimizing the composition and ratio of its alloys, enhancing oxidation resistance, order-disorder transition characteristics, magnetic transformation properties, and irradiation transmutation resistance, resulting in better thermoelectric reproducibility, high-temperature stability, and radiation resistance. While process temperatures in HTGRs are higher than those in pressurized water reactors, demanding greater stability and reliability from sheathed thermocouples, China currently lacks domestic nuclear-grade N-type sheathed thermocouples specifically designed for HTGR applications. In this instrument optimization study, the adaptability of N-type thermocouples to HTGR environments was analyzed, their manufacturing technologies were investigated, and China’s first prototype of domestic N-type sheathed thermocouple with nickel-based alloy cladding for HTGR engineering projects was developed. The sheathing material employs nickel-based high-temperature alloy Nicrobell C, which demonstrates excellent compatibility with the thermocouple wires. The elemental composition of Nicrobell C closely resembles that of N-type thermocouple alloys, with an additional 3% niobium to enhance mechanical performance under high-temperature conditions. This alloy maintains superior oxidation resistance and thermomechanical stability at temperatures up to 1 300 ℃, while its thermal expansion coefficient closely matches that of N-type thermocouple wires. The prototype underwent safety-grade instrument standard testing with the following key conclusions. The prototype passed all required tests including insulation resistance testing, radiographic inspection, metallographic structure examination of sheathing, thermal cycling testing at measurement junctions, graduation testing, long-term stability testing, and seismic testing in compliance with EJ/T 660-1992 and ASTM E235-2003 standards, meeting all technical requirements. The test results demonstrate excellent measurement accuracy and stability across the 0-1 200 ℃ range. This optimization study provides additional thermocouple selection options for temperature measurement design in HTGR projects, potentially offering more stable measurement accuracy and extended service life under high-temperature conditions. This advancement facilitates the localization of nuclear power measurement instruments and enhances China’s self-reliance in nuclear power equipment technology.