Abstract: The principle of least action,basic law of the nature and the final rule of the physics,was found to be followed by the explosive welding process by theoretical analysis and interface test. Namely the optimal welding interface can be obtained with the least explosive charge. The bonding energy of the interface can be looked upon the action on the course. To minimize the bonding energy,these rules must be followed such as the lower limit of explosive charge,the upper limit of span and the explosive of the lower critical explosion velocity. The principle of least action is achieved on the explosive welding process, and the bonding interface will be best.