The force closure properties of the underactuated forging robot grippers are analyzed. The gripping contact forces distributed between the interface of the tongs and the work-piece are considered as an equivalent resultant force whose contact model is friction point contact, and must meet force closure equations. Then the operation theory of N robot fingers to grasp an object can be used for the analysis of the force closure and the calculation of contact forces. Based on the configurations of heavy duty grippers, where the position distribution of the resultant contact forces is optimized, the iterative algorithm of linear constrained gradient flows is used to optimize the contact forces and calculate the gripping force. The force closure properties for different forging robots and in different operation conditions and mechanisms are analyzed. Simulation and experimental results demonstrate the effectiveness of the optimization method.