Abstract: In the two−way relay communication system, a new system performance optimization scheme is proposed. It is to solve the problem that the joint scheme of Physical−layer Network Coding and LDPC code coding algorithm with higher complexity and longer decoding operation time. Firstly, the joint communication system of Physical−layer Network Coding and LDPC code based on QPSK modulation is designed, the relay mapping scheme and the decoding algorithm of the system are given in this paper.Then, the factors such as code rate, code length and maximum iteration number of LDPC code are analyzed, those factors can influence Bit Error Rate of the joint communication system. Finally, the system decoding iteration scheme is optimized. The experimental simulation shows that the optimization scheme can ensure the system has good decoding performance, reduce decoding operation time and improve information exchange efficiency.