The log-likelihood ratio LLR is generally used for soft demodulation and operation, however, this method requires a large amount of calculations and consumes hardware resources, and is suitable for hardware platform FPGA programmable logic gate array. Aiming at this problem, this paper proposes a soft MAX computing method, which greatly reduces the complexity of LLR algorithm and has little effect on system performance. Finally, the Matlab is used for OQPSK soft demodulation simulation verification, and the test results show that the performance loss of the algorithm is very small compared with the traditional algorithm, which is suitable for hardware platform implementation.