Abstract: Addressing the problems concerning the spherical plug(material:Cr35Ni4Si cast steel) of the Φ130, 90 mm plug mill of the seamless steel pipe plant, including poor quality and short service life, etc., relevant application conditions and influencing elements are discussed. The manufaturing process for the plug mill spherical plug is optimized by means of taking actions like rimming the chemical composition of the plug material and additional heat treatment process. Relevant operation practice shows that thanks to these actions as taken, particularly the heat treatment process, the properties of the said spherical plug is significantly improved, and its service life is four times longer than before.