The influence of sky background radiation on bit error rate (BER) under on-off keying (OOK) modulation is studied, and the simulation method of sky background radiation received by the communication terminal is introduced. Simulation results show that when the communication BER is constant, the upper threshold of sky background radiation is related to the received signal optical power and the electrical noise of avalanche photodiode (APD). Direct solar radiation is the major background radiation received by atmospheric laser communication system, and atmospheric scattering,relative to the direct solar radiation, is very weak. When the solar zenith angle is small, the optic axis of the communication terminal can be adopted to avoid certain sun radiation angle, and it is ensured that the BER of the communication system is less than 10-9.