Abstract: The horizontal migration characteristics of convergence zone in complete sound fixing and ranging channel is studied by simulation model.The critical frequency of this phenomenon is derived according to Snell's law and Lloyd's mirror effect.It is found that when sound frequency is lower than critical frequency,the dominant mode of the convergence zone is frequency dependent.With the decrease of frequency,the convergence zone moves horizontally towards sound source,and the transmission loss increases significantly.When sound frequency is higher than the critical frequency,the dominant mode of convergence zone is almost frequency independent and the position and transmission loss of convergence zone will not change significantly with frequency.Most notably,the critical frequency varies with source depth.Critical frequency can be used to determine the frequency condition that need to be satisfied when convergence zone horizontally migrates at a specific source depth.The critical frequency formula contains source depth information which provides a good avenue for shallow target depth estimation in deep ocean.