Abstract: High-energy neutrons, gamma rays and electromagnetic pulses in the atmospheric environment, high-energy electrons and protons in the space radiation environment, etc., can cause changes in the properties of semiconductor materials, deterioration of device performance and even failure. The main reason for the failure of power devices caused by cosmic rays in the atmosphere is that high-energy neutrons cause single event burnout (SEB). Baihetan-Jiangsu ±800 kV UHVDC Transmission Project Butuo Converter Station is located at 27°N latitude, 102°E longitude, and an altitude of 2 500 m. The neutron flux is more than 10 times that of the horizontal plane, which will lead to a significant increase in the failure rate of the device. In order to verify that the failure rate of the 8.5kV thyristor in the project does not exceed 100FIT under rated conditions, a 200 MeV proton source is used for accelerated irradiation tests. Atmospheric neutron flux is used to calculate the total neutron fluence, and the effect of different voltages and temperatures on the failure rate of thyristors is compared and studied. According to the test results, the average failure rate of cosmic ray single event burnout of thyristor in high altitude area is calculated, which provides a basis for the design of the working voltage of the thyristor in the converter valve.