Abstract: This paper introduce the thermal hydraulic characteristic of the liquid in the high pressure safety injection dead lines, and analyze the mechanism and harm of thermal fatigue which may occur on these pipes. A modification is proposed for Daya Bay and Ling’ao Phase I nuclear power plants high pressure saft injection deadline,and take fatigue analysis on pipes added by the modification, find that there is no pipes added by the modification become fatigue after various produce condition loaded on it,demonstrate the efficiency and safety of the well modification.