3D spatial scale estimation is of great significance to 3D reconstruction, and is in demand in the real world in the case of a single image. Generally, the camera must be calibrated before the scale estimation. According to the perspective principle of the monocular image, a method based on two vanishing points and local scale information was proposed to calibrate the camera, thereby obtaining the estimation of the 3D scale information in the monocular image. Firstly, two orthogonal groups of parallel lines were selected from the monocular image to produce the coordinates of two vanishing points. Then the rotation matrix between the world coordinate system and the camera coordinate system was yielded using the vanishing point coordinate and the focal length information. The translation vector was also acquired through the property of vanishing point and the known local scale information. Finally, the 3D worldcoordinates corresponding to the pixels in the 2D image were restored to calculate the 3D scale information. Experimental results show that this method can effectively estimate the scale of building objects in a single image.