This paper studies the problems existed in the existing buildings transformation due to the influences of lacking original completion data, and introduces the method of combining reverse engineering technology, point cloud data processing technology and BIM technology together based on 3D laser scanning, to solve the problems mentioned above.The application of the method in project design and construction process has improved the feasibility of the design, as well as provided an effective solution to optimize and control the height of electromechanical installation of existing buildings.