The optical antenna of geosynchronous earth orbit (GEO) laser communication system exposes outside of satellite. Solar radiation affects the optical system seriously, and reduces available probability.The effect is more serious when the sun directly shine the primary mirror. The baffle can reduce the effect, but it is limited by the fairing′s size of rocket. By researching the characteristics of the GEO laser communication and analyzing the situation of primary mirror shined by the sun, a modified baffle with internal grids is designed to reduce the effect of the sun. Then the optical-device′s temperature and the available probability of the system combined the thermal control design are analysed. The results show that the available probability is improved from 93% with traditional baffle to 99.5% with the improved baffle. The GEO laser communication system can almost work all day with the modified baffle.