The performance of free space optical communication (FSO) system will deteriorate with the existence of light scintillating caused by atmospheric turbulence. But the performance of FSO can be improved effectively by means of diversity reception technology. To further analyze its impact on the performance of coherent receiving system, we use binary phase-shift keying (BPSK) modulation and heterodyne detection to establish FSO system model with spatial diversity over Gamma-Gamma atmospheric turbulence channel. Bit-error rate (BER) and outage probabilities (OP) of maximum ratio combining (MRC), selection combining (SC) and equal gain combining (EGC) with different numbers of receiving antennas and turbulence regions are analyzed. In addition, the BER and the OP of MRC, SC and EGC are compared with the performance of the traditional single-antenna receiving system with the same receiving aperture. The results show that spatial diversity based on MRC and EGC can improve the performance of coherent communication system over atmospheric turbulence, and the spatial diversity based on SC can improve the performance of the system only when the average signal to noise rate (SNR) is below a certain threshold.