In this paper an algorithm of reducing out-of-band emissions of OFDM based on GI is proposed. OFDM has better bandwidth efficiency and increased robustness to multi-path fading in wireless environment, but the out-of-band emissions of it are high. The algorithm based on guard interval can reduce the out-of-band emissions effectively by using the raised cosine windows. Of course the BER of systems change in a small range, which has been proved by the result of computer simulation.