Under the modulations of on-off keying (OOK), pulse-position modulation (PPM), differential pulse-position modulation (DPPM) and digital pulse interval modulation (DPIM), the intensity superposition coding multiple inpust multiple output (MIMO) optical communication system is compared and analyzed. With the improvement of modulation order, the results demonstrate that the latter three modulations have greater advantage than OOK modulation in the average transmission power, but the corresponding transmission capacity has declined. In the error performance, OOK modulation′s packet error rate is smaller than other three modulations in receiving low power. But when the received power is increased to a certain level, PPM, DPPM and DPIM modulations have the advantage of the 3~10 dB packet error rate to OOK modulation.