Since SNR is playing an important role in signal processing for OFDM, a kind of SNR estimation is proposed based on the guard interval. The channel model and the guard interval are analyzed. Firstly, two OFDM signals with different guard intervals are discriminated by using correlation function. Then, according to the structure characteristics of two different guard intervals, two kinds of SNR estimations are put forward. Finally, the SNR estimation is completed by combining the discriminant results with estimation algorithm. The simulation results show that the algorithm does not need auxiliary data and has good performance in different SNR conditions.