Abstract: 3D numerical models are always used in simulating tidal flow field of local waters， but the vast flow field beyond the simulated area by 3D numerical model can not be simulated. In order to solve this problem， a 2D and 3D nested model for the Yangtze estuary is developed based on a finite volume method with unstructured grids. The flow field of boundary cells between 2D and 3D is handled with an approximation method which supposes that the 3D horizontal velocity of the boundary cells can meet the multinomial distribution， and there will be some calculation errors in the boundary cells. The model is used for the Yangtze estuary in simulating tidal flow movement. The simulated results show that the tidal level as well as 2D and 3D tidal currents are well verified by the numerical model, and the accuracy of calculation can meet the requirements of the norms and specifications. So this method can be used to simulate the 2D and 3D tidal current movement of large scale waters.