Underwater wireless optical communication (UWOC) has the characteristics of high bandwidth and low latency, and have become a viable alternative for underwater communication. The transmitter and receiver arrays of this system are composed of six green light emitting diode (LED) and three photomultiplier tube (PMT), respectively, and forming a 6×3 multiple input multiple output (MIMO) transmission mode. This system achieved a 1 Mbps data transmission rate over a distance of 10 m water tank underwater channel. MATLAB was adopted to simulate this underwater channel model and predict the received optical power distribution on the receiving plane and the maximum value is -35.8 dBm. Moreover, the PMT anode output voltage and the cathode current waveform were tested. Derivation and calculation show that the signal-to-noise ratio (SNR) is 19.4 dB, and the bit error rate (BER) is approximately 1.1×10^-5. The theoretical minimum received power of the this PMT module can be as low as 1.5×10^-9 W, which reflects its extremely high sensitivity. Finally, through Monte Carlo channel simulation, the performance of MIMO is further verified, and the channel capacity of 35 bit·s^-1·Hz^-1 can be reached at a SNR of 25 dB.