Wireless optical communication technology is a hot-spot problem. In order to analyze the performance of optical communication system, the effects of pointing error and background light to the BER of the PPM system are analyzed with the Webb-Gaussian model of APD and they are also simulated based on Matlab. The result shows that the pointing error must be controlled under 0.15 beam width and the amount of background photons per PPM slot must be controlled fewer than 20 to achieve 10^-6 BER. At the same time, optimization of the APD's gain can depress the BER.