Abstract: Started with a brief introduction of the development of Computer Aided Process Planning(CAPP) system,this paper firstly included some new development trends of CAPP,such as integration,generalization and object-orientation.Then,the features of welded-structure assembly and welding process were analyzed,and how these features could affect the traditional design methods of CAPP system was explained.After all,according to the actuality of welded-structures CAPP system,the features of welded-structure processing and the trends of CAPP system,some general principles and methods of how to construct a practical welded-structure CAPP system had been brought forward.