Deformation extraction and magnitude is typically estimated by comparing three-dimensional laser scanner scans over the same area but at different time epochs. Although, these methods can easily be realized, the reliability of deformation extraction has not been evaluated. In order to improve the reliability of deformation monitoring, the point cloud error and registration error need to be analyzed in this paper, and to determine the deformation monitoring indicator. In order to avoid the influence of adjacent point error and the uncertainty of error space, the error entropy is introduced to the determination of cloud error space and thus the deformation monitoring indicator can be determined according to the relationship between error space and error extreme value. The feasibility of deformation monitoring indicator is verified according to the simulated experiment of plane board deformation. The method proposed by this paper is applied to assure the region and magnitude of the landslide.