Abstract: In orthogonal curvilinear coordinates, a numerical model of the three dimensional unsteady flow of the Yangtze estuary is presented. Boundary fitted grid, coordinate transformation, and finite difference , alternating direction implicit and moving boundary methods are adopted. The tidal levels and velocities are obtained by calculating the tidal currents in the estuary. The model reflects the flow variance in the complicated geographical conditions and the numerical results are in good agreement with the measured data.