The maximum punch pressure force (TMPPF) of cold welding pressure affects not only the performance of the welding bonding but also the amount of the area of welding bonding. If punch force is more bigger, it may be take place cracks, stress concentration and welding concave pits, which affect application of materials and increase incidence of faults. The process that punch pressure of cold welding pressure applied on sheet materials to attain welding bonding is simplified beam in cold pressure welding. Under normal circumstances it is an important method to TMPPF by the manner of experiment, but the method to attain TMPPF is limited. In the paper completely inelastic collision theory is applied to explain the process of cold welding pressure, the theory of completely inelastic collision offers principle theory to attain TMPPF. In this paper, two supposes are put forward, on the basis of two supposes the critical velocity is attained. At the same time, critical kinetic momentum or critical kinetic energy will be attained. On the basis of the law of conservation of energy, energy balance equation is attained, which will drop in calculating difficulty of the non-linear process of cold pressure welding and decrease calculated amount. In this paper on the basis of the process of collision all phases are analyzed. Indeed elastic-plastic deformation phase is completely inelastic collision phase, and from point of view of collision to calculate energy loss. At the same time, to suppose other plate is utter stiff can attain critical velocity, thus, to achieve the equation of TMPPF.