In order to study error influence of relaying nodes decoding process in which the signal to noise ratio is low, bidirectional soft information relaying scheme based on spatially coupled low density parity check (SC-LDPC) is proposed, and a more accurate model is also presented to describe the residual noise characteristics of the soft symbol. The proposed scheme adopts a pre-computing look-up table at the node of destination, which makes the calculation of the residual noise scale factor and the soft error variance more convenient. Compared with the previous soft noise modeling techniques, the proposed scheme reduces the signaling cost. In addition, the variance correction factor is introduced to correct the equivalent noise variance at the destination node. Simulation results show that, compared with other similar schemes, the proposed scheme achieves a significant improvement in bit error rate.