Experimental coal cores were collected from a coalbed of Sihe colliery and Zhaozhuang colliery, Qinshui Basin, China. Their gas effective permeability was studied under effects of water content and effective stress. The experiments were mainly carried out on a self-made "Triaxial Stress Thermal-hydrological-mechanical Coal Gas Permeameter." The results showed that when the temperatures of gas and coal were constant, a negative effect of either water content or effective stress was reported on the gas transportation, i.e., the gas effective permeability decreased with the increasing of water content under constant effective stress and it also decreased as the effective stress increased when the water content was constant. Under experimental conditions as in this study, the effects of water content and effective stress on the gas effective permeability was described by a linear-exponential equation, which presented that the gas effective permeability had a linear relationship with the water content and an exponential relationship with the effective stress. The permeation pores were defined as the primary places of transporting the coalbed gas. They were affected by water content and effective stress in different ways. The water content occupied the space of permeation pores, while the effective stress changed the shape of permeation pores. Consequently, the gas effective permeability was also affected by the two aspects.