Abstract: In order to study the hydrogeological conditions affected to the coalbed methane potential of the lignite reservoir and the accurate calculation of the water p ressure,the south part of Tuha Basin was selected for the study on the hydrogeological condition of the lignite reservoir and the gas control features of the hydrogeological condition were revealed. According to the water density affected by the quantitative factors,a calculated value of the water pressure was obtained with the coupling relation ship between the water density and the water pressure and a comparison was conducted on no consideration of the density varied value. The results showed that due to p oor water supply and runoff condition,the lignite reservoir at the south part of Turpan- Hami Basin would cause the underground water stagnation,high mineralization degre e 9 330 ~ 35 000 mg/ L and gas formation resistance of secondary organism. But in the deep depth area,the holding condition was good and could keep the primary or sec ondary biogas generated in the geohistory period. The water density was affected by the temperature,pressure,mineralization degree and others and thus there was certain difference between the varied water pressure value considered and the varied water pressure value not considered.