In order to know the function of downward boreholes and its gas drainage mechanism, gas content field in the goaf is simulated. The changing of gas content of roof tunnel gas drainage and roof tunnel with downward boreholes gas drainage is discussed. The results show that the range in 27m far away from coal face is the gas drainage short area of roof tunnel method, but the downward boreholes cover it. Downward boreholes like a barrier, which prevent gas from flowing to the upper corner in goaf. The gas content in area from coalface to 140m on the upper side of goaf is decreased by roof tunnel with downward boreholes, that reduces the value of gas content over limitation at upper corner. Furthermore, downward boreholes enlarge vertical fracture and realize gas to move to roof tunnel facility. Practical application reveals that the drainage gas is increased by roof tunnel with downward boreholes and gas content in the outlet tunnel is lowed. It is very important to realize high produce safety for gassy coalface.