Abstract: A numerical study of the effect of a solitary internal wave on the horizontal longitudinal coherence in a shallow water waveguide with a thermocline is carried out based on parabolic equation(PE) model.The result shows that the horizontal longitudinal correlation coefficient oscillates with time.The amplitude of oscillation is related to the depth of receiver and source,and the period of oscillation is related to the propagation velocity of solitary internal wave.A simple expression of the relation between horizontal longitudinal coherence and time is theoretically derived for the case that the propagation direction of solitary internal wave is parallel to the array.