Abstract: Coalbed methane is unconventional gas. Coalbed methane gas reservoir without obvious water interface, coal fracture and matrix is full filled with water, under temperature and pressure conditions, often rich water?soluble gas resources are accompanied by formation water. On the basis of previous research, this paper considers additional impact of dissolved gas from formation water, and established a new material balance equation. Further more, linearization the material balance equation and putting forward the average water saturation calculation method and cumulative coalbed methane production prediction equation, finally, getting the result that the material balance equation which considered water?soluble gas in coalbed methane gas is higher than that of not consider. Testing the reliability of the equation through real case, it can recognize the impact of dissolved gas form formation water to the nature gas in place quantitatively