Abstract: An apparatus working at super high pressure up to 840 MPa was designed.A sensor with simple structure,convenient usage,and high precision was designed and calibrated, with which the pressure in super high tank was directly measured.Meanwhile,the friction pressure between the moving sealing parts and the wall of super high tank was measured to be about 30% of the actual working pressure. In order to eliminate errors caused by the elastic deformations of the sealing part and the apparatus,different measuring method was adopted.The testing data were dealt with two mathematical models.The relationship between the compressibility of lubricating oils and the working pressure was established.The results show that the appatatus has reasonable structure, reliable sealing parts, and high measurement precision.