In this paper, we consider the ICI effect induced by time-varying channel. A simple and low-complex method is proposed to suppress ICI by windowing the received signal using Franks window coefficients in time domain and combining the extended signal interval which was not interfered by multipath to the corresponding information part of the cyclic-prefix (cp) based OFDM system. The ICI are suppressed by equivalently reducing the channel time-variation according to the useful extended signal interval. If the channel time-variation is linear in one OFDM symbol and the useful extended signal interval period as long as the information part period, the equivalent channel time-variation of information part will turn into a constant which means no ICI occurs. And then, a N-point fast Fourier-transform (FFT) and one-tap equalizer is used for demodulation and channel equalization. The complexity of the proposed method is only N log N. The analysis of Franks window coefficients for equivalent channel mitigation in linear time-varying channel are investigated and simulation results are provided to validate the performance of the proposed method.