A boring machine for cylinder comprises a frame (1), guide rails (29) arranged below the frame (1) for adjusting back and forth, and spindle parts arranged on the frame (1). The boring machine is characterized in that lugs (33) are designed on both sides of the frame (1); an adjusting and fastening mechanism (25) is also arranged on the frame (1); a draw bar (24) is arranged on the adjusting and fastening mechanism (25) and is connected with a slide base (13) of the spindle parts; a transmission spindle (14) in the spindle (11) is fixedly connected with a second bevel gear (9) arranged in a cutter holder (38) on the front segment of the spindle (11) at the end; a sliding table (7) for cutting tools is arranged on the cutter holder (38) and is connected with a second screw (10); and the second bevel gear (9) is engaged with a first bevel gear (8) on the second screw. The boring machine can process welded flange hole with saddle-like beveled edge on a cylinder.