When the tappet is pushed up by the cam, the reaction force of the push rod acting on the support seat 5 and the plunger 3 makes the plunger overcome the force of the spring 8 and move downward relative to the tappet body 1, so the lower cavity of the plunger The internal oil pressure increases rapidly, so that the check valve 7 is closed. Due to the incompressibility of the liquid, the entire tappet rises like a rigid body, thus ensuring the necessary valve lift. When the oil pressure is high, a little oil will leak out through the fitting gap between the plunger and the tappet, but this will not affect the normal work. Similarly, when the valve is heated and expanded, the plunger also moves axially relative to the tappet body due to the pressure, and squeezes the oil from the lower cavity through the above-mentioned gap. Therefore, when the hydraulic tappet is used, the valve clearance can not be left, and the valve can still be closely connected with the valve seat when it is heated and expanded.