Abstract: The formations of shale gas and tight gas are abundant in the nanopores of shale and tight sandstone, and the pores are partially saturated with water under reservoir conditions, so the evaluation of gas flow capacity by considering water saturation is important to gas production forecast. Based on the thermodynamic equilibrium theory between liquid and vapor in formation nanopores, we quantify the distribution of reservoir pores water under ultra-low water saturation condition, and establish the relation between water saturation and gas phase permeability. Results show that the impacts of water saturation on the gas flow can be divided in two forms: (1)bound water film in large pores will reduce the e ective pore diameter for gas flow; (2)capillary water in small pore will block the entire gas flow channel. And the e ect of water saturation on gas flow capacity cannot be ignored. In case that water saturation is 20%, the gas flow capacity will reduce 10%, compared with the dry conditions. Under the condition that water saturation is 40%, the gas flow capacity will be reduced by about 20%. The results calculated by the models in this paper are in agreement with the experimental results in a newly-published article.