Abstract: The paper mainly introduces how to get the parallel image from the lean image using the conditions of geometry and restriction in the surface of building according to the theory of perspective transformation. After transformation, the principal distance of rectified image has the same value as one of original image. Then, using the least control condition, the parallel images can be magnified to the scaled image. The technology proposed in paper made getting facade image with large scale rather rapidly, not influenced by the height of building, and can be used in many kinds of mobile mapping situation.