Abstract: Due to the variation of target's orientation or the fluctuation of sea surface,acoustic signal will spread in frequency while propagating in the sea.Such a channel is called a frequency-spreading channel.The performance of a matched filter will be lowered under such circumstances,and the optimal detector will be the segment replica correlator(SRC).The calculation speed is the bottleneck in SRC implementation.Two fast algorithms based on FFT are proposed and their calculation abilities and memory capacities are discussed.Numeric simulation results show that the two algorithms can achieve more accuracy as well as faster speed.