Abstract: To improve the reliability and the safety of the telecommand lock circuit, in view of the development process and technical characteristics of the satellite products, a procedure of reliability analysis for the satellite key circuit is proposed to resolve the problem of instability when the lock circuit is in the temperature environmental test. The stability of the circuit parameters is analyzed quantitatively under different conditions, the design margin of the circuit's important parameters is used to determine the optimum design, at least 67% of the margin under the acceptable worst conditions of application is ensured. It shows that the parameter's tolerance range is consistent with the design under the actual worst case, so the reliability and the safety of the telecommand lock circuit are fully guaranteed.