For space-ground optical communication, computational formulas for maximum acquisition time, mean acquisition time and acquisition probability were presented which were used by the multiplex raster spiral method. The simulation model of acqusition were established, the relation between the acquisition time and acquisition probability were analyzed, and effects which caused by the relative velocity of air communication platform to acqusition simulation were also disscussed. The simulation results show that when acqusition area of uncertainty of communication terminal is 50 mard, scanning overlap factor is 0.12 and signal-to-noise ratio of acqusition detector is more than 6, the acquisition probability of space-ground optical communication system is better than 95%, and the maximum acquisition time is about 36 s and the mean acquisiton time is about 12 s.