The computational fluid dynamics (CFD) method is employed for numerical simulating the flow field in the bit which can be opened and closed with large via. Analysis was made on the speed field under different flow conditions, the optimum drilling air volume is acquired and the “obstruction vortex” is found in the flow field. And through the analysis on the pressure field under the optimal flow conditions, the existence of “suction pressure difference” is found in the flow field. The influence mechanism of “obstruction vortex” and “suction pressure difference” to the heat dissipation and slag discharge is realized, which can be the references to the optimized design of the bit and will lay a foundation for the popularization and application of the technology of gas drainage with screen pipe protection for the whole borehole in soft outburst coal seam.