To reduce the influence of fast-varyi ng fiber channel on reliability and validity of coherent optical orthogonal freq uency division multiplexing (CO-OFDM) system,we present a new channel estimation method based on analysis of symbols mapping (ASM) ap plied to this fast-varying fiber channel.This method analyzes the constellat ion diagram of the received OFDM symbol,and then the channel′s transfer function is modified cons tantly.That is to say this method can fast track the change of fast-varying fiber chan nel,and at the same time,this way doesn′t need a large number of pilots for channel estimation.Thro ugh simulation,we can show that in 40Gbit/s fast-varying fiber channel CO-OFDM system,while the signal-to-noise ratio (SNR) is 10dB and system′s bi t error rate (BER) is 10－3,the pilot interval of ASM algorithm is reduced half compared with the conventional zero forcing est imation (ZFE) al gorithm.While the pilot interval is 10(every ten OFDM symbols inset o ne pilot) and system′s BER is 10－3,compared with conventi onal ZFE algorithm,ASM algorithm has 3.5dB SNR improvem ent.