For the orthogonal frequency- division multiplexing (OFDM) systems in time- variant channels,traditionalestimation methods are of poor estimation performance and high computational complexity.In order to solve theseproblems,we propose a channel estimation method named as DD- LMMSE (Dual- Diagonal Linear Minimum MeanSquare Error).In this method,a system model is constructed on the basis of expansion model (BEM) and the esti-mated coefficients of BEM are obtained by performing the diagonal LMMSE twice,and thus achieving the channelimpulse response (CIR) in time domain.Simulation results show that,whether in slowly time- variant or rapidlytime- variant channels,the proposed method has better performance and lower computational complexity.