Abstract: Described here in the article is the process for manufacturing the P11 large-sized,light-wall seamless steel tube with the Φ720 mm rotary expanding mill. And analyzed are the main aspects of the said tube,including the chemical composition,the geometric dimensions,the microstructure and the properties,etc. It is regarded via the analysis that the chemical composition and all the properties of the P11 tube are in compliance with relevant requirements as specified under Standard ASTM A 335/A 335M;the microstructure is homogeneous,and the brittle transition temperature is approximately -43.5℃;and the extrapolation 540℃stress rupture strength is 66.1 MPa,a high value that conforms to appli- cable ASTM requirement.