Variation modeling is one of the most important tools for assembly variation analysis. Considering dimension and geometric precision, and part situation precision, the deviation source that affects assembly accuracy is clustered into two types: deviation of geometric location and orientation, variation of geometric form. And these different types of variation or deviation were unified by Virtual Fixture. So a 3D rigid assembly state space modeling technique was developed for stream of variation analysis (SOVA) in multi-station processes. A missile-cabin-assembly process was analyzed in this model. It enhances the applicability in modeling complex assemblies. The developed methodology outperforms the techniques based on other simulations in computation efficiency. It is more powerful not only in forward analysis of complex assembly systems, but also in backward analysis. The model is validated by using Monte Carloseries simulations.