Abstract: The reactor coolant system of high-temperature gas-cooled reactor (HTGR) is a closed helium loop. The design pressure of helium in the primary circuit is 7.0 MPa, and the design cold-end helium temperature is 250 ℃. The mass flow rate of primary circuit helium is one of the essential thermal-hydraulic parameters for implementing reactor protection functions and calculating reactor thermal power. Limited by the internal structure of the HTGR nuclear island, there are no suitable pipelines for installing traditional flowmeters such as orifice plates and Venturi tubes. In the 10 MW high-temperature gas-cooled test reactor, an indirect method is adopted to determine the primary circuit helium mass flow rate. In commercial HTGR nuclear power plants, nuclear safety-class elbow flowmeters are applied for primary circuit helium flow measurement. Elbow flowmeters have a relatively narrow measuring range, delivering high accuracy at high flow velocities but failing to ensure satisfactory accuracy at low flow velocities. Thermal gas mass flowmeters feature a wide turndown ratio and can maintain favorable measurement accuracy even at low flow velocities. To apply thermal mass flowmeters to the flow measurement of high-temperature and high-pressure helium in the primary circuit of HTGRs, the research on key technologies including high-temperature thermal probes and theoretical correction models for thermal flowmeters used in high-temperature and high-pressure helium flow measurement was conducted in this paper, and a prototype thermal mass flowmeter suitable to high-temperature and high-pressure helium measurement was developed. Compared with conventional thermal sensors, high-temperature thermal sensors were improved in material selection and manufacturing processes. The sensor package adopts a ceramic skeleton around which platinum resistance wires are wound. It is fabricated via secondary sintering after glazing, and magnesium oxide powder is filled directly between the sensor and the stainless steel sleeve to fill gaps and enhance thermal conductivity. The probe body, fixing flange and thermal probe are all made of 316L stainless steel, and the internal wire connector is ceramic. The thermal probe and the probe rod were joined by socket welding and flaw detection tests were performed on the welds to ensure no leakage. Both the probe rod, and the platinum resistor were verified in accordance with the calculation method specified in ASME BPVC-Ⅲ ND 3133 for components subject to external pressure. The calculation results show that the wall thicknesses of both the probe rod and the platinum resistor meet the requirements for a design pressure of 8.1 MPa. The high-temperature helium thermal flowmeter adopts an overall split design, consisting of a measuring assembly, split cables and a transmitter box. The sensor has a design temperature of 350 ℃, a design pressure of 8.1 MPa, an accuracy requirement of ±1.5%F.S, and outputs an active 4-20 mA current signal. The thermal flowmeter can collect gas flow data in the pipeline in real time. Relevant tests and verifications were carried out on the prototype thermal mass flowmeter for high-temperature and high-pressure helium measurement. The test results indicate that: The high-temperature thermal probe passes high-temperature and high-pressure test verification and is competent for helium measurement under working conditions of 7.0 MPa and 250 ℃; The coefficients to be calibrated in the theoretical correction model for thermal mass flow measurement of high-temperature and high-pressure helium are sensitive to temperature but less affected by different medium compositions and pressures; After calibrating the model coefficients using atmospheric air as the medium and incorporating compensation based on the physical property parameters of the actual gas under measurement conditions, the calibration curve of helium under 7.0 MPa and 250 ℃ can be fitted, which satisfies the flow measurement demands of high-temperature and high-pressure helium in HTGRs. This study proposes a new approach for primary circuit helium mass flow measurement using thermal mass flowmeters.