Because of rapidness and easy to update, raster map is important in GIS and digital mapping. In many circumstances, the projection that users demand is not according with the original projection of the raster map, thus the projection transformation of raster map is necessary, Raster map projection transformation, which is essentially image transformation, involves transformation among four coordinate systems: original image coordinate system, original projection coordinate system, new projection coordinate system, new image coordinate system. In this paper, compared with the vast computation and the slow speed of traditional transform method, a rapid algorithm of raster map projection transformation based on dual transformation is researched. In this algorithm, first of all, transform rectangle control grid strictly according to the formula of projection transformation; and then, aiming at each rectangle, using the four corners to construct dual linear transformation polynomial between rectangle image and corresponding quadrilateral image in original raster map; finally, the direct transformation between image coordinates of two maps is realized. The experiment has proved that this algorithm not only satisfies precision demand, but also greatly improves transformation speed.