In the digital detection of parts which Computer Aided Design (CAD) model is known,to compare and analyze the CAD model and the measured point cloud model of the parts,at first,the problem that the CAD model dispersed into point cloud data must be solved. Aiming at solving this problem, a method based on UG NX secondary development was proposed. The secondary development tools of UG NX and their basic characteristics were developed, and used them and Visual Studio 2010 development environment to design the dialog of CAD model discretization in UG NX10.0. The principle and basic process of CAD model discretization were expounde,programmed the callback function in dialog box, realized the discrete function, and obtained the point cloud data. The discrete results are compared with the original CAD model,and concluded that the discrete results can be well represented the original CAD model.