The present invention relates to a valve train mechanism and a fuel supply system of internal combustion engine, and particularly relates to an oil control device of a hydraulic fully variable valve system of the internal combustion engine. This device connected with the hydraulic valve system of internal combustion engine, the device consists of a housing, a rotary valve, a hydraulic accumulator and a transmission mechanism. The rotary valve, the hydraulic accumulator and the transmission mechanism are installed in the housing. The rotary valve consists of a rotary valve shaft and a rotary valve sleeve. The hydraulic accumulator consists of an accumulator piston, an accumulator spring, an end cover, a sealing seat ring and a rubber gasket and is installed in the cavity at one end of the housing. An accumulator chamber is provided between the rotary valve and the hydraulic accumulator. The transmission mechanism consists of a transmission gear, a gear shaft and a cross slide coupling. The transmission gear is installed on the gear shaft, while the gear shaft is connected with the rotary valve shaft through the cross slide coupling. The present invention can replace a high-frequency solenoid valve, it is applicable to use with hydraulic fully variable valve system of single-cylinder and multi-cylinder internal combustion engines. And the present invention has the advantages of simple structure, reliability, good manufacturability and low cost.