Virtual hand is an important medium for implementation of human-computer interaction in the process of virtual assembly.And the comparability between the action of virtual hand and real man-hand has always been regarded as one of the important criteria to judge whether the virtual environment has the property of immersion.On the basis of analyzing the relative position,posture,and motion behavior relationship between virtual hand and the object to be operated during virtual assembly,this paper proposed an implementation method of combining grasp index with collision detecting technology for identifying grasp action of virtual hand.The method is to identify grasp/quit using grasp index and to implement joint angle adjustment through simulation cyclic iteration and joint angle interpolation calculation.Although some traditional method identifying the grasp action on the basis of the shape of the object is operated,the simulation result getting from virtual environment created by EON STUDIO shows that it has good features of serviceability and fidelity to realize operation control of virtual hand.