Abstract: Based on the characteristics of power generation and thermodynamic cycle of the mark-1 fluoride salt cooling high temperature reactor system, the thermoelectric conversion efficiency of the FHRs coupled with air Brayton cycle system is studied. By studying the gas turbine calculation method, the effect the air flow rate and inlet air temperature on FHRs and the variation laws of FHR efficiency under non-rated conditions are analyzed. The results show that the thermal efficiency is up to 42.6%, and the efficiency decreases with the increasing of the air flow rate. Less than 5% change of air flow rate can keep the efficiency changing within 2%, and different reheating strategies of the system can be selected if the change of the flow rate exceeds the scope. In addition, the thermal efficiency of the system decreases with the increasing of the air inlet temperature, and the change of the system efficiency is less than 1%.