Abstract: The gas emission is large in No.4321 fully mechanized coal mining face in Jinneng Mine, and the gas emission sources are mainly from the goaf. Aiming at the the problems of roof's complex nature and poor drainage effect of high level borehole, the characteristics of roof rock crack is researched by using the numerical simulation software-ABAQUS. Simulation result shows that rational fissure development zone is located in the place from air gate horizontal distance 18 ~ 24 m and above roof 20 m. So, drainage parameters of high level borehole is designed and optimized on account of this result.