The invention discloses a precoding method of a multi-user bidirectional MIMO relay system based on an incomplete channel, and belongs to the technical field of wireless relay communication. The method comprises the following steps: firstly, respectively calculating total signals received by a kth information source and a kth user in two time slots; in a non-ideal channel state, establishing a channel model; then, according to the system model and the channel model, constructing an optimization problem expression of a transceiving precoding algorithm of the MIMO relay system; solving a kth signal source receiving filtering matrix W1, k and a kth user receiving filtering matrix W2, k; optimizing the kth information source precoding matrix B1, k according to the maximum power constraint condition; optimizing a relay forwarding matrix; optimizing a kth user precoding matrix through a square constraint quadratic programming problem; and finally, carrying out joint iteration until convergence to obtain an optimized pre-coding matrix. According to the algorithm, non-ideal channel state information is considered, the algorithm can be more suitable for an actual communication system, and the performance of the system is effectively improved.