Abstract: A mathematical model for the velocity field of vertical jet discharging into tidal flows is presented,in which the κ-ε turbulent model is adopted for turbulence and the "motion"-rigid-lid approximation of the free surface is described.According to the oscillation feature of tide,a track-while-scan line by line method for solution of the discretized equations is developed.Numerical calculations are done on the unsteady velocity fields in different periodical cases.The predicted relative height of the major recirculation region is in good agreement with the experiment date.Hence,the effect of unsteadiness is also discussed and some conclusion remarks are given.