Abstract: The bit error rate(BER) of the space optical communication system was calculated versus two kinds of general data modulation.In the course of the calculation, avalanche photodiode(APD) is used as the optical detector,and different values of the bandwidth,background power and the load resistance of the optical detector were taken into account.The curves of BER as a function of received optical power were acquired.Result shows that all the bandwidth,background power and the load resistance of the optical detector have a great deal of effect on the BER.The BER increases with the bandwidth and the background power,and decreases with the resistance of the optical detector.