Abstract: Investigated are the causes for occurrence of wall thickness inhomogeneity of the steel pipe as rolled by the mandrel mill,involving four main aspects,i.e.,billet heating schedule,process adjustment,rolling tools and cooling water. And accordingly,preventative actions are proposed. The actions include a billet heating scheduele that can not only satisfy the production cycles, but also properly control the heating temperature and heating speed so as to restrain the trend of grain uneven enlargement, and thus get the thickness inhomogeneity restrained; considerations of process adjustment and proper operation of the rolling tools of the piercing mill to prevent helical wall thickness inhomogeneity of the pipe in in case that no defect stays with the billet-heating temperature, and considerations of process adjustment, proper operation of the rolling tools, and cooling water of the piercing mill to prevent straight line wall thickness inhomogeneity of the pipe.