To reduce the influence of fiber dispersion and noise in coherent optical orthogonal frequency division multiplexing(CO-OFDM) system,we present a new channel estimation method based on impulse response processed with window(IRW) algorithm.This method relies on least square(LS) estimation,and then uses the channel impulse response processed with the time domain window to filter the noise outside the window to ensure the length of channel impulse response in OFDM guard interval.The IRW method is not more complex than LS algorithm,and has lower bit error rate(BER) than LS algorithm by nearly an order of magnitude.The simulation results show that in the 256-subcarrier CO-OFDM system,and with the same BER of 10-4 the required signal-to-noise ratio(SNR) in IRW algorithm is lower than that in the LS algorithm by nearly 2.5 dB,while only 0.7 dB higher than that in the linear minimum mean square error(LMMSE) algorithm.At the same time,the number of complex multiplying of IRW algorithm is only 8 times higher than that of LS,but nearly 32 times lower than that of LMMSE.