It is very difficult to calculate the large scale reservoir storage rapidly and accurately by the traditional calculating method of reservoir storage, and the result can't meet the request of the reservoir regulation. On the basis of the spatial information technology, this paper presents a new method of calculation of reservoir storage by gathering the precise terrain data of the reservoir and establishing reservoir watercourse DEM. From the method, the software system of reservoir storage calculation has been developed and used to calculate the static reservoir storage and dynamic reservoir storage of the Three Gorge Reservoir. The calculation results are quite accurate, which indicates that the method is feasible and can be used for the other large scale reservoirs that have the terrain DEM data.