Abstract: Steam generator (SG) tube rupture overfill in a nuclear power plant have been analyzed with thermal-hydraulic system code, which validates that SG do not overfill under this accident. Expansion research of the overfill analysis for SGTR also has been conducted in this paper, considering multiple cases for tube break, including single tube double ended, multiple tubes double ended and tube small break. The analysis results of three cases are compared and the most conservative situation is provided in this paper. The results show that the case of single tube double ended is the most limit case, under which the SG overfill do not occur, and the overfill margin is less than that in the other two cases.