Continuous phase modulation is a kind of modulation with constant envelope, which has high frequency spectrum utilization and power utilization. Its constant envelope property can be well adapted to nonlinear amplifier modulation. So continuous phase modulation can be used in satellite communication, deep space communication and other communication fields. In order to improve the system performance, a system model combining serial cascade code and continuous phase modulation is used. Due to the high complexity of the traditional decoding algorithm, the improved Max-Log-APP algorithm is used for decoding. In the additive Gaussian white noise channel, a parameter optimization method is designed to solve the problem of the correlation between decoding iterations and CPM parameters. The results show that under the condition of low SNR, the system performance can be improved about 1dB by optimizing the bit error rate.