A method of improving the mass ratio of DTH hammer and impact hammer by large cavity structure is presented to increase the drilling efficiency of DTH hammer, and LS-DYNA dynamic analysis method is applied to conduct the simulation analysis on the large cavity structure of DTH hammer bit’s rock fragmentation efficiency and stress state. The results show that the large cavity structure can reduce the mass of DTH hammer bit and adjust the mass ratio of DTH hammer bit and the impact hammer, which can obviously improve rock fragmentation efficiency. The mass ratio is changed from 0.79 to 0.91, the biggest absorptive impact energy change rate in the processes of rock collision and after the collision were 41.13% and 67.17% respectively. The forces of two kinds of large cavity DTH hammer bits meet the fatigue strength requirements for long-term operation. The numerical simulation calculation results show that this structure of DTH hammer bit is feasible in the construction.