Abstract: Because of the problem of the Minimum Mean Square Error with Successive Detection (MMSE-SD) algorithm with high computational complexity for OFDM systems in the presence of frequency and time double selective channels, an improved low-complexity MMSE-SD algorithm is proposed in this paper. The algorithm extends firstly the channel matrix and detection matrix, then establishes the relationship between them by using recursive inversion instead of direct inversion at each detection. Both theoretical analysis and simulation results show that the improved algorithm retains the performance of the original algorithm with much lower computational complexity compared to the original MMSE-SD algorithm; Compared to other algorithms, the improved algorithm balances the performance with the computational complexity. The larger normalization Doppler frequency shift, the better performance it possesses with the unchanged computational complexity.