Satellite-borne opto-electromechanical instruments preserve both optical systems and mechanism properties; the former has a strict requirement for temperature, and the latter has a complex structure and is constantly moving. This is a challenge to the test validation of the thermal characteristics. A case study of the indirect validation method of the thermal design for the optical communication terminal is presented. The indirect validation method may be utilized to predict the in-orbit temperature through the thermal analytical model, which is corrected using temperature data in the thermal balance test. The thermal balance test and model correction have been made. Very good correlation was demonstrated between the computed results of the proposed model and the measured test data; a deviation of 81% is less than 5 ℃. Thus, the model is appropriate for use in validation of thermal design. Comparing the thermal analytical results to the flight results, the deviation of 81% is less than 4 ℃. This demonstrates the accuracy and the effectiveness of the indirect validation method, and the method meets the application requirements of optical communication terminals on satellites. The proposed indirect validation method greatly reduces the difficulty of thermal testing and is of certain reference value to altitude-varied and high temperature precision opto-electromechanical instruments.