Abstract: In view of the problem of water existing in the drilling and coal rock in the process of measuring coal seam gas pressure, the influence of water on the direct method of measuring coal seam gas pressure was analyzed from two aspects of macro effect and micro effect. The macro effect was mainly caused by the gravity of water, which can be corrected by measuring amount of water in the drilling after testing; the microcosmic effect was analyzed by laboratory experiments on gas displacement in coal with water. The experiment shows that:water can replace the gas in coal micropores, so that the adsorbed gas becomes free; in the unsaturated state, when the water content in the coal is between 2% and 10%, with the increase of water content, the gas replacement amount also increases, and the final equilibrium gas pressure also increases; from the perspective of displacement rate, the initial replacement rate rapidly increases to the maximum, then enters the attenuation stage after reaching the peak, and the attenuation trend conforms to the exponential attenuation law; according to the law of gas desorption after pressure relief, the amount of gas desorption of coal sample decreases with the increase of coal seam water content. Considering the uncertain factors such as the volume of the pressure chamber, the wetting range of water to coal and the water content of coal, the quantitative study on the influence of water on gas pressure can not be carried out.