Abstract: Underwater visible light communication (UVLC) system with light emitting diode (LED) as transmitter has a low transmission rate due to the limited device bandwidth and it is greatly affected by water environment quality that makes the communication comprehensive performance poor. In order to improve the channel capacity and communication quality, the channel multiplexing technology into the LED-UVLC system was applied. The FPGA was used as the data processing core, LED blue and green light emission array and avalanche photodiode (APD) receiving array were built, and a 4×4 multichannel UVLC system was designed. We placed the system in an underwater environment with a length of 20 m and achieved bidirectional transmission with a bit error rate (BER) of approximately 10⁻⁷ at a communication rate of 26.7 Mbit/s. The experimental results show that when transmitting data files of different sizes, compared with the traditional single input single output (SISO)-UVLC system, the communication time is saved about 4 times and the channel capacity gain is consistent with the simulation. In addition, with the same water quality and communication distance, the system can achieve better BER performance.