Normal pressure shale gas plays such as the Ohio Formation Shale and Marcellus Formation Shale in the Appalachian Basin in the U.S. have been developed commercially. Although Wufeng-Longmaxi Shale in southeast Sichuan Basin in China has been initially developed, its production efficiency is not obvious and its cost is high due to the large burial depth and poor reservoir properties physical properties. In the normal pressure shale gas formation, the absolute pressure changes at different depths, which influences the gas occurrence state, and then has a significant influence on gas content and mobility. Therefore, it is urgent to carry out quantitative research on the differences of normal pressure shale gas reservoir conditions, gas occurrence state, gas content and mobility between China and the US. In this study, the Wufeng-Longmaxi shales of Well-LY1 in Pengshui of China and the Marcellus shale and Ohio Shale of Appalachian Basin in the United States are selected as the research objects under normal pressure. Based on the volume method, and with the considering of the influence of the temperature, pressure, maturity, water and oil on the adsorption capability and maximum adsorption capacity, and the temperature and pressure, porosity and water saturation on the determination of free gas volume, the gas content of three groups of shale reservoirs is evaluated. The reservoir pressures are considered as the initial pressures and the simulation are conducted based on a pressure drop of 5 MPa. On the basis of clarifying the desorption process of adsorption and free gas in the depressurized production respectively, the fundamental cause of the difference in normal pressure shale gas production between China and the U.S. is revealed. Compared to the Marcellus Shale in Appalachian Basin, the lower adsorption capacity, porosity and free gas of the Wufeng-Longmaxi shales result in low production of adsorbed gas and free gas. However, compared to the shale of the Ohio Formation, the deeper burial, higher temperature and higher pressure in the Wufeng-Longmaxi shales result in the extremely low recovery rate of adsorbed gas. Its lower porosity also contributes to the lower free gas production.