The combined effect of atmospheric attenuation, turbulence, pointing errors, and spatial correlation on the bit error rate (BER) of an atmospheric wireless optical multiple input multiple output (MIMO) system is investigated. The maximum likelihood detection criterion of the pulse position modulation (PPM) under the combined effect is derived via a Poisson counting model and an exponential correlation model. On this basis, the upper bound of the BER of the atmospheric optical MIMO system is derived via the Wilkinson approximation method. In addition, a simulation experiment is used to further analyze the influences of the various factors in the combined effect on the BER of the atmospheric optical MIMO system. The results show that when the atmospheric attenuation is constant, the spatial correlation has the most serious impact on the BER of the system, followed by the jitter error and the atmospheric turbulence. In contrast, the transmission distance is the most important factor influencing the BER of the system when the atmospheric attenuation is changeable. In addition, the performance improvement of the system resulting from the spatial diversity only works well when the channels are independent, and this improvement is not obvious when the channels are correlated.