Rock compressibility is an important parameter in gas reservoir engineering. It affects the results of gas reservoir numerical simulation. At present, there are many calculation methods of rock compressibility. In the mine field, the most commonly used method of calculation is Hall Plate empirical formula. But there is a wide gap between its results and the actual. The Hall Plate empirical formula and the elastic modulus method are comparatively analyzed, and the impacts of different calculation methods on gas reservoir numerical simulation results are studied. If rock compressibility is calculated by the elastic modulus method, its results are relatively reasonable. But if it is calculated by the Hall Plate empirical formula, the value of rock compressibility is too large. That’s to say, it overvalues the elastic energy of formation rocks. And the velocity of pressure drawdown of the gas reservoir numerical simulation is caused to be too slow.