<p>Compared with twist drill and gun drill, the particularity of Boring Trepanning Association (BTA) deep hole drilling is that there are many guide pads embedded in the drill bit. The guide pad on the BTA bit not only has the function of supporting and guiding the hole, but also has the function of extrusion on the processed surface of the hole wall. The repeated extrusion of the guide pad on the hole wall makes the machined surface have different surface characteristics from the ordinary cutting process. In this paper, the change of the machined hole wall surface roughness, microstructure, and microhardness of low alloy steel was studied. Compared with the machined surface after cutting, there were still typical surface defects after extrusion, but the number and type of defects were significantly reduced, and the degree of surface grain refinement was significantly improved. Compared with the raw material before processing, the average grain size of the surface layer after extrusion was reduced from 1.08&#xa0;µm to 0.55&#xa0;µm. The above processing technology also caused an increase in the depth of the hardened layer and the increase in the surface hardness value, which was caused by the dislocation and fine grain strengthening effect during the extrusion of the surface.</p>