1. A system for detecting defects of objects based on a CAD (Computer-Aided Design) platform, the system comprising a measurement apparatus, an application server, and a database server, wherein the application server comprises: a data acquiring module for acquiring point cloud data of an object measured by the measurement apparatus and acquiring a three-dimensional digital model of the object stored in the database server; a point cloud cutting module for cleaning and editing the point cloud data by cutting overlapping areas of the point cloud data; a point cloud filtering module for filtering the point cloud data to make the point clouds smooth and regular; a data aligning module for aligning the point cloud data with the three-dimensional digital model of the object; and a data comparing module for comparing the point cloud data with the three-dimensional digital model of the object, obtaining any deviations in x-y-z Cartesian coordinates, and indicating different deviations by using different colors.