The vertical turbulent mixing of the isodensity oscillation current in tide estuary has been studied in this paper, On the basis of the governing equation and character analysis of the tidal flow, the expression of time-space distribution of shear stress, which is influenced by gravity and inertia, has been derived. Considering the terms of gravity and inertia, the theoretical equation of turbulent exchange coefficient of the tide estuary flow has been developed. Then the vertical average and the time average equation have also been obtained .To verify those equations, the time-space distribution of vertical turbulent exchange coefficient in Yangtze Estuary has been calculated according to measure data. Its variation character with the flood and ebb tide has been analyzed.