The invention relates to an automatic clamping and loosening device for a workpiece on a lathe. The automatic clamping and loosening device comprises a three-jaw chuck and a tool which is connected with the three-jaw chuck, wherein the tool is provided with at least two clamping oil cylinders and an oil filling port; an oil path is arranged between the clamping oil cylinders and the oil filling port; the clamping oil cylinders are communicated with the oil filling port through the oil path; and the other end of the oil filling port is cooperatively connected with an external oil filling device through an oil pipe. The invention also relates to a clamping and loosening method. After the workpiece to be clamped is arranged on the tool, the external oil filling device is cooperatively connected with the oil filing port through the oil pipe, hydraulic oil is pumped into the clamping oil cylinder through the external oil filling device, and the workpiece is clamped by the clamping oil cylinders under the action of high-pressure hydraulic oil; and after the workpiece is machined, the hydraulic oil in the clamping oil cylinders is recovered by the external oil filling device through the oil path, and the workpiece is loosened by the clamping oil cylinders. The automatic clamping and loosening device and the clamping and loosening method have the advantages of easiness and convenience in operation, low production cost and extreme suitability for large-scale production, promotion and application.