1. A method for fast volume rendering of 3D ultrasound image, comprising steps of: dividing a view plane into first-level grids arranged vertically and horizontally according to a predetermined size; calculating gray-scale values of pixels on four vertices of each grid by projection; comparing deviations among the gray-scale values of pixels on four vertices of each grid with a predetermined threshold, wherein if the deviation is smaller than the predetermined threshold, the grid is determined as a flat grid, and if the deviation is larger than the predetermined threshold, the grid is determined as a non-flat grid; filling each flat grid by obtaining gray-scale values for all pixels inside the flat grid through interpolating the gray-scale values of pixels on four vertices thereof; wherein for the non-flat grid, the dividing step, the calculating step, the determining step and the filling step are performed repeatedly until the non-flat grid is subdivided into atomic grids so as to calculate gray-scale values for remaining pixels by projection.