The optimal beam divergence angle of FSO communication system is discussed in this paper.According to the spatial optical communication link equation,the communication link margin increased with the beam divergence angle reduced.Under the misalignment error,the relationship between offset angle,beam divergence angle and link margin is analyzed.Assuming that the misalignment loss is the only factor affected the link margin,the maximum offset angle expression is derived.When the unaligned(offset) angle reaches its maximum value,the angle is optimal beam divergence angle.The simulation results show that the optical power decreased with the deviation angle increased.when the beam divergence angle of the system is 1.6 mrad,it is the optimal beam divergence angle.This research provides theoretical support for the uplink and downlink communication link power calculation.