The invention relates to an SCMA code book design method, an SCMA coder and an SCMA system. The SCMA code book design method comprises the following steps: (1) fixing the minimum Euclidean distance, minimizing the basic volume of a parent constellation, and generating a parent constellation; (2) optimizing the parent constellation with the minimum product distance as an objective function to get an optimal rotation angle; (3) decomposing the parent constellation into three sub-vectors, and rotating the three sub-vectors at the obtained optimal rotation angle to get the rotation sub-vectors of the parent constellation; and (4) rearranging the coordinates of the rotation sub-vectors to get an SCMA code book applicable to six users. The method can help the system to get good bit error rate performance. Based on a component interleaver generated at a channel transmitter and a component deinterleaver at a channel receiver by the SCMA coder, the bit error rate of the downlink physical layer of a 5G communication system under a Rayleigh fading channel is reduced, and high resistance to communication channel fading is achieved.