Impact rotary compacting drilling is a kind of advanced soil anchor hole forming method, two-way pneumatic DTH hammer is the main equipment used for it. Its critical device is two-way pneumatic impact mechanism; the gas distribution stroke design is related to the DTH hammer conventional working performance and realization of effective reverse impact. Based on the analysis on DTH hammer gas distribution theory, using section calculation method and finite difference principle, the study was made on the design of gas distribution stroke length of two-way pneumatic impact mechanism and put forward the design reference value of gas distribution stroke length ratio.