Abstract: According to the definition of rock pore compressibility and rock pore volume compressibility, this article defined rock volumetric strain, developed the theoretical formula of rock pore compressibility and rock pore volume compressibility of formation pressure drop, proposed a new method to calculate the rock compressibility of formation pressure drop, obtained rock porosity change data in the formation condition from the experimental data in the surface condition by simulating effective pressure increase experiment, and calculated the rock pore compressibility and rock pore volume compressibility changing rule of formation pressure drop in the example gas reservoir. The calculated results using this new method coincide well with the traditional method.