Abstract: A linear pre-coding scheme is proposed for the amplify-and-forward(AF) multiple-input multiple-output (MIMO) relay system with channel estimation errors and feedback delay based on the minimum mean squared error (MMSE) criterion. In the proposed scheme, the channel feedback delay and the channel estimation errors are considered in the source-relay link and the relay-destination link, then the closed form of the optimal source precoding matrix, the relay precoding matrix and the decoder matrix are deduced with the maximum power constraint at the source station and the relay station based on the MMSE criterion. Numerical simulation results show that the proposed pre-coding scheme can effectively improve the performance of bit error rate (BER) and mean square error (MSE).