Abstract: For thermal control performance of missile-borne phase array antenna with built-in heatpipe/PCM thermal control device, a mathematic model is established. The PCM’s phase transformation process is simulated by the effective specific heat method based on the DSC test data. Besides, the influences of the PCM’s thermal conductivity and the number of fins on the thermal control performance of the device are studied, focusing on the highest temperature on the TR modules’ mounting surface and the PCM’s melting process in the storage. It is shown that both a higher value of the thermal conductivity or a larger number of fins are conducive to improving the performance, and the two groups of parameters are complementary in effect. A prototype is built according to the optimum parameters, the actual test shows that the heatpipe/PCM thermal control device provides a good solution to the antenna’s thermal design problem.