An oil injector comprises an oil injector assembly, an oil injection assembly, a functional assembly and a holding assembly, wherein the oil injector assembly is provided with a spring and an oil duct for receiving fuel oil; the oil injection assembly is provided with a lubricating nipple body which is provided with a cavity containing a valve needle and an oil duct supplying the fuel oil to the cavity, the valve needle is provided with a first end and an opposite second end, and the first end and the lubricating nipple body form a hydraulic seal; the functional assembly is arranged between the oil injector assembly and the oil injection assembly and provided with a pin and a structural base for receiving the pin, the structural base is provided with an oil duct communicated with the oil duct of the oil injector assembly and the oil duct of the lubricating nipple body, a first end of the pin abuts against a second end of the valve needle, an opposite second end of the pin receives spring thrust, at least one part of the pin is in hydraulic seal contact with the structural base, the valve needle and/or the lubricating nipple body are/is provided with oil grooves/an oil groove guiding the fuel oil in the cavity to the first end of the pin, and the oil grooves/the oil groove enable/enables the fuel oil to reach the first end of the valve needle and then reach the first end of the pin after oil injector is opened; the oil injector assembly, the oil injection assembly and the functional assembly are held together in a hydraulic seal mode through the holding assembly. Therefore, pressure in opening or closing can be conveniently adjusted.