The work aims to investigate the wear properties of Ti6Al4V titanium alloy tool in gun drilling and discuss the effect of tool wear on drilling axial force. Deep hole drilling test was designed and each holes was drilled to 575 mm. After drilling a hole, the tool was analyzed for wear properties and wear values by focus microscope and the dynamometer was used to extract the axial force signal. The tool wear form was analyzed through observation by microscope and the tool wear grade was given according to actual wear of tool. Based on the analysis of the axial force, the influence of tool wear on the axial force of drilling was studied. From the tool wear curve, the wear process could be divided into three stages during the whole drilling test: initial wear, normal wear and severe wear. The average wear amount and the maximum wear amount of the first flank face on the outer blade were always greater than that of the rake face in the three stages. When the drilling depth reached 11 m, the overall wear rate of the tool increased, and the tool entered the severe wear stage. When the drilling depth reached 14 m, the maximum wear of the first flank on the outer edge increased rapidly. In addition, the axial force curve showed that the initial wear stage remained unchanged, the normal wear stage increased steadily and the severe wear stage tended to be stable. The main wear forms of tool are burning wear and adhesive wear on the rake face and the first flank of the outer blade, breakage and tripping on outer edge and side edge, peeling of large area on guide surface. Thus, pits are formed. These three cases can cause the failure of tool. In the severe wear stage, the tool wear rate increases rapidly and the cutting force is large. Therefore, the tool should be re-ground before the severe wear stage during actual process.