This paper presents a method to calculate the contact force and deformation during the gripping process with over-constraint. Considering the distances between workpiece and gripper at the contact point are variable, the relative configuration of workpiece and gripper is described. This paper solves the indeterminate contact force resulting from over-constraint of workpiece-gripper system based on linear elastic model. A simulation result is present to illustrate the deformation and overall displacement of workpiece during the grasping process. This calculating method can be utilized to design and optimize heavy-duty gripper.