It is very important to measure the porosity in situ of overpressure deep reservoir for characterization and prediction. The normal method with back-pressure valve technology for exerting pore pressure and measurement of flow was not able to gain ideal porosity precision because of low control precision of pore pressure and low measurement precision of pore volume. A new measurement method for overpressure porosity was developed with the employment of double high precision constant pressure and speed pumps to exert pore pressure and measurement the increment of the pore volume. Firstly, the pore pressure was built by the double high precision constant pressure and speed pumps located both sides of rock sample. Secondly, an empirical relation between volume variation of dead volume and effective pressure was gotten through a set of calibration measurement with the employment of steel plug. Thirdly, the pore volume measured under the normal condition was adopt as basic pore volume. At last, the pore volume/porosity can be measured by the basic pore volume and the increment between the volume variation of bump and dead volume calculated through the empirical relation. The measurement results of 5 rock samples shows that the new method has good repeatability and matches the porosity measured under overburden pressure good.