Abstract: In order to overcome the channel fade effect caused by the atmospheric turbulence, the free space optical (FSO) system model with spatial diversity was analyzed based on intensity detection pulse position modulation (PPM) in weak turbulence atmosphere. The calculating formula of slot error rate (SER) of the system without diversity was derived under PPM firstly. Then as a benchmark, under the case of independent and identical distribution, the average slot error rates of three linear combining technologies, i.e., the maximal ratio combing (MRC), equal gain combining (EGC) and selection combining (SC) were compared by means of numerical simulation. The results show that the improvement of system by MRC is the best, followed by EGC and SC is poor. However, SC is simpler and more convenient. The technology of spatial diversity receiver combination is efficient to improve the performance of FSO and has strong ability of resistance to atmospheric channel fade, and is suited for optical wireless communications systems.