Abstract: A joint design scheme of rate compatible modulation (RCM) based on polar codes is proposed to improve the spectrum utilization rate of wireless communication. Correspondingly, an efficient joint iterative decoding algorithm based on belief propagation (BP) and soft cancellation (SCAN) is designed. According to the algorithm, the stability and timeliness of the decoding process can be improved by optimizing the information iteration method of the variable node log-likelihood ratio (LLR) and adopting the improvement measures of limiting the upper limit of decoding symbols. Finally, compared with the scheme of RCM based on the distance-optimized cascade low-density parity-check codes(LDPC), the proposed efficient joint iterative decoding algorithm has lower decoding complexity under low signal-to-noise ratio conditions, and has better throughput and bit error rate performance. Therefore, the scheme proposed in this paper is suitable for wireless transmission under harsh channel conditions.