Based on the model of the spatial coherent optical communication system under the influence of the atmospheric turbulence and the plane-wave propagation model of aperture averaging, by using of numerical simulation, under the weak irradiance fluctuation condition the influence of aperture averaging including the atmospheric turbulence inner scale and outer scale is investigated on the bit-error rate and the optimum receiver aperture diameter of the coherent optical communication system. The results show that aperture averaging can decrease the bit-error rate effectively; the improved effect of aperture averaging on the bit-error rate is more obvious for the higher original SNR, the shorter transmission distance, the longer wavelength, the larger value of the phase compensation mode J and the receiver aperture diameter which is closer to the optimum value; aperture averaging affects the optimum value of the receiver aperture diameter, and if the value of the phase compensation mode J is larger, the influence is more obvious; the bit-error rate and the optimum receiver aperture diameter will increase with the increase of atmospheric turbulence inner scale and decrease with the decrease of atmospheric turbulence outer scale. This will provide the necessary theoretical basis for the design of a coherent optical communication system.