in order to strengthen the protection of ancient buildings and avoid "secondary damage" to ancient buildings, gls-2000 3D laser scanner collects the information of ancient buildings, uses improved ICP algorithm to realize the point cloud data splicing of ancient buildings, Geomagic software to realize the point cloud data denoising and simplification, and Revit software to extract the feature lines and 3D modeling of ancient buildings. Finally, taking the roof components and the gallery components of ancient buildings as examples, the application effect of bim scanning technology is analyzed, and the application effect of the improved ICP algorithm is tested. The results show that bim scanning technology can accurately determine the size information of ancient buildings, establish 3D model of ancient buildings, and effectively preserve the information of ancient buildings. The registration time of the improved ICP algorithm is 5.623 seconds, the registration accuracy is 4.21 λ / mm, the average error of point cloud 3D model is 0.113 m, and the mean square error is 0.154 M,and the application effect is good.