Abstract: Bottom hole pressure (BHP) has a great effect on CBM production, so it's necessary for us to calculate BHP accurately during CBM production. The mathematical models of BHP calculation were developed based on mass conservation law and energy conservation law, considering the influence between water production and BHP during the flow process. Matlab7.11 was used to write solving program. And the relationship among operational parameters and their effects on deliverability were analyzed. The results show that the coupling effect between water production in coal seam and BHP make them not follow a linear relationship, especially when reservoir permeability is good and coal seam is thick. High gas production rate can not only reduce pressure drop losses in the ring, but is convenient for CBM transportation on the ground.