Channel capacity is the maximal transmission ratio of the communication system. It is one of the important indexes to measuring system communication ability. The channel model of the optical multi-in and multi-out (MIMO) system is established, then the ergodic channel capacity and outage capacity of optical MIMO system are analyzed by combining pulse position modulation (PPM). And then, the in-closed approximate expressions of ergodic channel capacity are deduced with and without fading in the case of no background radiation, respectively. It shows that the channel capacity of optical MIMO system can be doubly raised under the condition of same sending power and bandwidth. Its feasibility is confirmed using Monte Carlo simulation experiment.