A special driving cam-based combined engine valve driving device, characterized by comprising: a special driving cam, located on one side of a positive work cam of an engine, and having a base circle portion and driving lift bosses positioned on the base circle portion; a valve bridge, provided with a secondary piston slidably installed in a secondary piston hole, an actuated valve being connected to the secondary piston; a driver, fixedly installed on a rocker shaft and provided with a master piston configured to cooperate with the special driving cam, the master piston being slidably installed in a master piston hole; and a driving oil circuit, allowing the master piston hole and the secondary piston hole to be in intermittent fluid communication with each other by its own on- and off-states, the driving oil circuit being in fluid communication with an oil supply line by means of a positioning pressure control unit, wherein in a state where a positive work rocker of the engine does not drive the valve bridge to displace, the driving oil circuit is in the on-state, and the master piston hole is in fluid communication with the secondary piston hole; at the same time, in a state where the oil supply line supplies oil to the driving oil circuit, when the special driving cam rotates to allow the base circle portion to be in sliding or rolling fit with the master piston, the master piston stretches out until contacting the base circle portion under the hydraulic action of the driving oil circuit; when the special driving cam rotates to allow the driving lift bosses to be in sliding or rolling fit with the master piston, the positioning pressure control unit cuts off the driving oil circuit and oil supply line, and a hydraulic linkage is formed between the master piston and the secondary piston, so that the driving lift bosses can drive the displacement of the actuated valve connected to the secondary piston by means of the master piston; in a state where the positive work rocker drives the valve bridge to displace, the driving oil circuit is disconnected along with the displacement of the valve bridge, the hydraulic linkage between the master piston and the secondary piston is released, the engine oil in the secondary piston hole leaks out, and the secondary piston resets in the valve bridge; the oil supply line provides one-way oil supply to the driving oil circuit through the positioning pressure control unit; and wherein the driver is provided with a main elastic element, configured to drive the master piston to retract during the pressure relief in the driving oil circuit. a special driving cam, located on one side of a positive work cam of an engine, and having a base circle portion and driving lift bosses positioned on the base circle portion; a valve bridge, provided with a secondary piston slidably installed in a secondary piston hole, an actuated valve being connected to the secondary piston; a driver, fixedly installed on a rocker shaft and provided with a master piston configured to cooperate with the special driving cam, the master piston being slidably installed in a master piston hole; and a driving oil circuit, allowing the master piston hole and the secondary piston hole to be in intermittent fluid communication with each other by its own on- and off-states, the driving oil circuit being in fluid communication with an oil supply line by means of a positioning pressure control unit, wherein in a state where a positive work rocker of the engine does not drive the valve bridge to displace, the driving oil circuit is in the on-state, and the master piston hole is in fluid communication with the secondary piston hole; at the same time, in a state where the oil supply line supplies oil to the driving oil circuit, when the special driving cam rotates to allow the base circle portion to be in sliding or rolling fit with the master piston, the master piston stretches out until contacting the base circle portion under the hydraulic action of the driving oil circuit; when the special driving cam rotates to allow the driving lift bosses to be in sliding or rolling fit with the master piston, the positioning pressure control unit cuts off the driving oil circuit and oil supply line, and a hydraulic linkage is formed between the master piston and the secondary piston, so that the driving lift bosses can drive the displacement of the actuated valve connected to the secondary piston by means of the master piston; in a state where the positive work rocker drives the valve bridge to displace, the driving oil circuit is disconnected along with the displacement of the valve bridge, the hydraulic linkage between the master piston and the secondary piston is released, the engine oil in the secondary piston hole leaks out, and the secondary piston resets in the valve bridge; the oil supply line provides one-way oil supply to the driving oil circuit through the positioning pressure control unit; and wherein the driver is provided with a main elastic element, configured to drive the master piston to retract during the pressure relief in the driving oil circuit.