Abstract: When drilling through the fractured formations of a shale gas reservoir, the formation pressure will exhibits a significant difference from normal pressure as well as after fracturing. It is hard to obtain accurate formation pore pressure in fractured formations of shale gas reservoirs by using traditional pore pressure prediction methods. To tackle this problem, we analyze the causes of dynamic changes in formation pressure in fractured formations under different operating conditions. We also analyze the large increase of formation pressure after shale gas reservoirs stimulation. Based on those analyses, we propose a method of calculating pore pressure change in both naturally fractured and stimulated shale gas reservoirs. Applying the method shows it has asimple principle and clear mechanism, and gives results which highly agree with those from field tests. As an effective method for accurately calculating dynamic pore pressure in fractured formations of shale gas reservoirs, this method offers high practical value.