Sparse code multiple access (SCMA) is a novel non-orthogonal multiple access technique, which enjoys the low complexity of message passing algorithm (MPA) and the shaping gain of multidimensional codewords. The performance of SCMA is usually evaluated by the system level simulation and represented by the block error rate (BLER) as a metric. In this paper, we propose a new evaluation method to evaluate the performance of SCMA at the system level. We establish a link to system (L2S) interface and get the relationship between the instantaneous SNR and effective SNR. The simulation results show that the system level BLER achieved by the proposed method is accurate for various coding rates and modulation schemes.