Abstract: Based on low communication rate requirement of underground visible light communication, an underground voice transmission system by visible light between mobile caplamp and hydraulic ram LED base station was designed by use of pulse code modulation mode. In the system, voice signal is coded by TP3067, and the coded signal is added direct current signal to drive LED. The received coded signal by receiver is decoded to renew the voice signal after photoelectric detection and amplifying. The experimental results show that the system error rate is 10-83 when receiving power is -9 dB?m under system communication rate of 2.048 Mbit/s, which realizes voice transmission without distortion.