Polar coded sparse code multiple access (SCMA) system is conceived in this paper.A simple but new iterative multiuser detection framework is proposed,which consists of a message passing algorithm (MPA) based multiuser detector and a soft-input soft-output (SISO) successive cancellation (SC) polar decoder.In particular,the SISO polar decoding process is realized by a specifically designed soft re-encoder,which is concatenated to the original SC decoder.This soft re-encoder is capable of reconstructing the soft information of the entire polar codeword based on previously detected log-likelihood ratios (LLRs) of information bits.Benefiting from the soft re-encoding algorithm,the resultant iterative detection strategy is able to obtain a salient coding gain.Our simulation results demonstrate that significant improvement in error performance is achieved by the proposed polar-coded SCMA in additive white Gaussian noise (AWGN)channels,where the performance of the conventional SISO belief propagation (BP) polar decoder aided SCMA,the turbo coded SCMA and the low-density parity-check (LDPC) coded SCMA are employed as benchmarks.