Abstract: Most of the continuously-cast round billets used by domestic tube makers are produced by the “BOF + LF” process or the “EAF + LF” quick process which result in occasional occurrence of surface porous defects of the billet with morphological characteristics obviously other than the common surface porous defects such as pinhole, blister and interior porous hole of the cast billet. Brief analysis is made on the development mechanism and apparent characteristics of the surface porous defects of the continuously-cast round billet, based on which control measures for such defects are proposed.