LaCaSrMnOis a thermochromic material which can be used as thermal control device. However, its solar absorptivity is too high for that in spacecraft application. To reduce its solar absorptivity, an optical thin film is designed in this paper by using simulated annealing genetic algorithm. This film can effectively reflect the solar radiation at the short wave and can be transparent at long wave. A designed optical thin film is deposited on the surface of thermochromic material by electron beam evaporation. Experiments show that the solar absorptivity is reduced from 0.78 to 0.28 at short wave, and the transmissivity is 0.87 at long wave. The results match pretty well with the theoretical predictions in a global view.