To obtain the in-situ porosity of rock, a transient gas seepage model with main parameters of porosity and permeability is developed from an in-situ steady-state permeability test. A porosity inversion model is constructed based on genetic algorithm. Rock permeability and transient pressure decline data obtained from in-situ permeability tests are adopted as known variable and observational data respectively. Then a new method is formed to determine rock porosity inversion. The effect of porosity results from a typical site on the pressure decline curve and the feasibility of porosity inversion is analyzed. A parallel porosity inversion program is conducted, and then the porosities of rock at two holes are inversely calculated using the mainframe computer. Finally, the correctness of inversion results is verified by numerical simulation and laboratory measurement. As the in-situ test method has small damage to rock and its pore structure, the results of porosity by inversion method has better area representation than those by laboratory measurement, so that the inversion result can be applied to numerical simulation of gas leakage in rocks.