Abstract: The transmission time of acoustic wave between transmitter and receiver needs to be estimated in underwater acoustic positioning.Because of the influence of transmission distance,the amplitude of receiving signal varies greatly,which is unfavorable for signal detection.Base on the correlation detection of liner frequency modulated pulse signal,a method of adaptive threshold setting is proposed,which can replace the automatic gain control circuit in some conditions.This method can set threshold of correlation detection automatically and easily,according to the parameters of signal,channel,transmission system,receiving system,and the arrival time of signal.Simulation and experiment indicate that this is an effective method of calculating arrival time of acoustic signal.