Aiming at the problems of high cost and complex algorithm of conventional coherent receiver in optical access network, a low cost simplified coherent receiver(SCR) signal processing method is proposed. The transmitter uses four-level pulse amplitude modulation(PAM4) high order code to modulate the signal, and the receiver uses unbalance compensation algorithm and feedback full digital clock recovery algorithm to process the output signal, and calculates the bit error rate through frame synchronization and code element decision. The simulation and experimental results show that the proposed method has good performance in the system. When the optical power of the input signal is -9 dBm, the bit error rate of the system is 10-4, which is lower than the bit error rate decision of the hard decision forward error correction(HD-FEC) threshold.