A coherent orthogonal frequency-division multiplexing passive optical network (OFDM-PON) system based on optical frequency comb (OFC) and injection locking local laser is proposed. Optimum OFDM signal power, optical local oscillator (LO) power and injection ratio are found by both theoretical calculation and experiments. The bit error rates (BER) performance of three kinds of downlink transmission are experimentally compared in which LO is provided by semiconductor optical amplifier (SOA), wide-linewidth slave laser and narrow-linewidth slave laser after 25 km single mode fiber (SMF) transmission. The results show that injection locking can improve the receiving sensitivity without affection by the inherent linewidth of the slave laser, and provide coherent light for uplink transmission. Therefore, the proposed system can reduce cost and provide the possibility for the application of the optical coherent detection technology in access networks and data-center interconnections.