The National Aeronautics Administration of China is planning lunar exploration. The National Key Laboratory of Tunable Laser Technology of Harbin Institute of Technology is responding to this challenge by developing a optical communication system with pulse-position modulation signaling and APD Receivers. So in this article, we mainly calculate the bit-error rate (BER) of detector uncoded pulse-position modulation signals using a Webb-Gaussian modeled avalanche photodiode (APD) receiver in the presence of additive Gaussian thermal noise for the lunar-earth laser link. Performance curves for 4-ary pulse-position modulation obtained through Monte Carlo simulation as well as a numerical calculation result are presented. The result indicates that the number of background photons per slot must be restrained below 20 in order to achieve 10-6 BER when the number of signal photons per signal slot is 120.At the same time, the pointing error is the important factor that influences the BER .It must be restrained under 15urad when the signal optical beam width is 100urad in order to achieve 10-6 BER. And we find that optimized the APD gain can improve the performance of the optical communication system. The number of background photons per slot can be more than 20 when BER need to be the order of 10-6 if the number of signal photons per signal slot is 120.If we take the effect of atmosphere on the optical pulse shape into account, the BER will rise. In order to improve BER, we present the analysis of return-zero (RZ) pulse position modulation scheme for the optical communications with non-rectangular pulses. The results show that RZ PPM will restore performance losses due to reduced peak intensity during the detection process.