Abstract: The in-situ measurement formula for rock permeability is derived based on the theory of steady gas seepage. A rough judgement method for Darcy seepage in engineering is given based on the Pavlovsky’s Reynolds number formula. An in-situ system to measure rock permeability is established, which consists of gas source subsystem, gas mass flow control subsystem, sealing subsystem and gas pressure measurement subsystem, and the components and functions of each subsystem are introduced. The measurement flow and steps are given. The complete method of in-situ measurement of rock permeability based on pneumatic tests in boreholes is formed. The in-situ pneumatic tests in boreholes are performed in granite medium at a certain site. The in-situ permeability of field holes is got, and some key aspects in the tests are analyzed, such as pressure course in boreholes, judgment of Darcy seepage, influence of pneumatic mass flow, influence of stopper’s expansion pressure and representative area of measurement data. The results show that the stability of this system is good, and the measurement data are reasonable and have a large representative range. The proposed system can be applied to the in-situ measurement of rock permeability.