In order to solve the problem of long water drainage period with low efficiency in No.15 coalbed methane of Qinshui basin and to avoid the effect of the aquifer in the K2 limestone above the No.15 coal bed on CBM development，horizontal well was applied for gas recovery by water drainage in roof water of the coalbed methane reservoir to enhance coalbed methane recovery efficiency. Flow mechanism model was established by numerical simulation. Effect of gas recovery by water drainage on the No.15 coal seam and K2 roof limestone and its influencing factors were researched. The simulation results show that the technique can significantly reduce the time of the water drainage and pressure drop and increase the cumulative gas production. The location of horizontal well in the roof，the angle between horizontal well and vertical fracturing crack and horizontal section length are related to CBM production. Water drainage intensity is related to the gas breakthrough time of CBM. The longer the horizontal section length is，the higher the gas production is. The production effect of the horizontal well that is located in the middle of the K2 limestone is better than that of the well located in the bottom.When the angle between horizontal well and the vertical fracturing crack is 45 degree，the gas production effect is the best.The water drainage intensity of horizontal well has a great influence on the time of gas breakthrough，and less influence on the cumulative gas production.