The invention relates to the technical field of a mold closing mechanism, and provides a volume supplementary type mold locking device adopting equivalent volume replacement and having wider using adaptability. The device comprises a movable template, a fixed template, mold locking oil cylinders, a supplementing oil cylinder and a communicating and stopping valve, wherein each mold locking oil cylinder comprises a cylinder body, a piston and a piston rod. Hydraulic oil in the supplementing oil cylinder is used for supplementing volumes of parts occupied by the piston rods in rod cavities, real equivalent volume replacement is realized, that is, the hydraulic oil is not required to return to an oil tank during mold closing, oil suction from the oil tank is not required during mold opening, so that the energy consumption is low, the pressure rising time is short, the reaction is fast, and the device has the advantages of energy saving and high efficiency; the device is simple in structure, small in size, high in mold opening, closing and locking performance and precision, uniform in stressing and stable and reliable to use, and is applicable to various specifications and models of direct-press type mold closing mechanisms.