Abstract: This paper gives an experimental method which improves the hydrostatic pressure in the present piston-cylinder type apparatus. Manganin gauges were made successfully and calibrated accurately by phase change of Bi. Measurement and analysis of both strain and relative displacement all show that (1) there are no synchronous contracting role of hoops in the range of 0.000 1~3.08 GPa when the pressure cavity is empty or full of liquid; (2) large difference between upper and lower oil pressure will bring about danger of high pressure explosion. In order to obtain the hydrostatic pressure higher than 3.08 GPa. We suggest to modify the criginal design: (1) if the internal hoop and the middle hoop combined into one and the outer hoop divided into an upper half layer and a lower half layer, strain of hoops will be enhanced; (2) adjusting the area of contact surface between hoops will help the synchronous contracting role; (3) increasing the unsupported area of sealed sleeve will reduce the difference between upper and lower oil pressure, hence will be advantageous to safety of operation.