Abstract: Described here in the article are the designs of the large-sized，heavy-wall 4130M-QT-125K mechanical structure seamless steel tube，involving chemical composition，and processes of steel-melting/casting，piperolling and heat treatment，etc.Also summarized are the control tips formanufacturing the said tube.Relevantoperation practice demonstrates thatmanufacturing process as designed for the large-sized，heavy-wall 4130M-QT-125K mechanical structure seamless steel tube is reasonable，and the tube itself is in possession of such characteristics as high controlled-dimensional tolerance，consistent strength and toughness，yield strength：R_t0.65under 890~1 020 MPa，tensile strength：R_m≥960，after-fracture elongation：A≥18%，reduction of area：Z≥50%，impact energy absorption at 0℃：≥60 J，shearing area rate：≥80%，and full wall thickness Rockwell hardness：33~37 HRC. All the property values are up to or even over relevant agreed requirements.