Abstract: Mechanism motion simulation based on virtual assembly is a very important step to products assembly motion in virtual environment. Mechanism motion simulation after finishing assembly simulation in virtual environment can provide effective reference data for analyzing the mechanism performance so as to analyze whether the mechanism is logical and improve the quality of assembly. First of all, combined with the development of mechanism motion simulation both home and abroad, the essential idea and the whole frame of realizing the mechanism motion simulation based on virtual assembly technology are presented in virtual environment, and the geometrical method which is utilized to solve the problem and calculate the kinematic parameters of mechanism motion simulation is proposed by taking an example. Geometrical method divides one mechanism into some basal groups so as to analyze the kinematics regularity of these basal groups instead of analyzing the kinematics regularity of the whole mechanism. Secondly, the key algorithms of automatic recognition of kinematic pair, automatic search of mechanism and mechanism identification are discussed. These algorithms have important significance for reducing the workload of pretreatment of mechanism motion simulation after finishing assembly simulation in virtual environment so as to promote the level of automatization of simulation effectively. Finally, all the methods are implemented and validated in the prototype system the Virtual Assembly Process Planning (VAPP).