Virtual assembly environment, which shows an opening system where products are assembled, is very important for product design. With virtual reality technology being more and more mature,virtual assembly becomes an important research direction on its applications and also provides a new way for assembly-oriented design and evaluation. Firstly, current virtual assembly environment and assembly constraint conditions are outlined in this thesis. Combined with the theory of virtual assembly environment, the theory of assembly constraint conditions is analysed. Secondly, the assembly modules of CATIA are introduced. The classes and modules of CAA which related with assembly constraint and assembly environment are described. Lastly, eight virtual assembly constraint conditions were developed by CATIA/CAA, which included fix constraint, angle constraint, parallelism constraint, contact constraint, perpendicularity constraint, tangency constraint, offset constraint and coaxiality constraint. A part of an aircraft was assembled by using the virtual assembly environment, and good results are obtained.