In order to contrast the properties of diverse thermal control coatings, Fluent is used to simulate the thermal balance of small radiators in the near space at a height of 20 km. Respectively, S781 white coating, S956 gray coating and La_1-xSr_xMnO₃ compound thermotropic materials are used as the thermal control coatings of the radiator. The UDF (User defined function) code is employed in setting the emissivity of the thermal control coating which varies with the change of temperature. Through the simulation, the temperature distribution of radiators in two extreme conditions, vertical solar radiation and no solar radiation, is obtained. The simulation results show that the temperature of the lithium battery of the radiator using the La_1-xSr_xMnO₃ compound as a heat-control material is within the optimum working temperature range under the vertical solar radiation. But its difference of the power's average temperature in two situations is 8.89 K, larger than that of S781 white coating 6.57 K. During the simulation, the emissivity of La_1-xSr_xMnO₃ thermochromic material has a small change of about 0.11, and the emissivity of the highest temperature in heat dissipating region can reach 0.8 in vertical irradiation condition, while 0.69 at the lowest temperature in no solar radiation circumstance.