The utility model discloses an oil cavity for measuring the oil volume changes, which comprises a cylinder with bottom, a cylinder-shaped lid, a connecting cylinder and a fastener of the lid which form the cylinder body; the cylinder body is divided into two cavity by a rolling film, the bottom of the upper cavity is fixed with a sleeve which is coaxial with the cylinder, a piston rod is positioned in the sleeve with which is sliding matched, the top of the piston is fixed at the central of the cap-shaped piston, the two side of the rolling film stick the upper surface of the cap-shaped piston and the plate respectively, the displacement sensor is installed on the cylinder bottom where the position is symmetry to the sleeve, the measuring steel wire of the displacement sensor is connected with the inner top face of the piston, the signal wire of the displacement sensor is leaded out of the cylinder body, the lid has an opening for the oil liquid in the central. The utility model has a new structure, which using the length of wire of the displacement sensor changes with the change of the oil volume to get the volume change directly by the electrical signal, which is convenient and has high accuracy.