In order to realize the thermal control of high-power Focal Plane Apparatus (FPA),the thermal design of FPA was studied according to the structure characteristics of the FPA by simulation and experiment.The main requirements of the thermal design of high-power FPA were introduced and the feature of thermal design of a focal plane assembly was analyzed.Then,the thermal performance was simulated by means of code TMG.A thermal test was carried out and the temperature fit curve of FPA according to the thermal test was given.Finally,the method for choosing the thermal control mode rationally was put forward according to thermal design example.The results show that the temperature of 50 W high-power FPA can be controlled to lower than 35℃ when it works for 2 min continuously in 12℃ cool source.It can meet the design requirements of FPA.