Reliability estimation of a thermal imager is becoming increasingly important in infrared weapon systems. The reliability estimation of a thermal imager based on prior information is studied using reliability expectation values and exploring experimental data as the prior information and a posteriori information, respectively, to evaluate the reliability of the thermal imager during development. The findings indicate that the method for estimating thermal imager reliability using prior information is both reasonable and feasible. This approach lessens the dependence on large field test sample sizes as typically required by classical statistics, thereby reducing resource costs through smaller field sample sizes.