Abstract: A depth joint algorithm on joint separation-decoding is proposed for low density parity check （LDPC） code single-channel co-frequency modulated signals. The log likehood ratio (LLR) of variable nodes is alternatively updated by the single symbol by symbol steps of decoding or separation, the LLR magnitude is small and it is easy and timely to rectify by separation steps. Decoding will work after the bit error rate below the threshold, and the algorithm convergences finally. Simulation results show that the convergence threshold is declined and the performance is enhanced by uniting a single step of separation and decoding while the traditional algorithm cannot be convergent, and good tolerances of the proposed algorithm towards noise are shown for 8PSK mixing signals when bit error rate is 10^-3.