Abstract: It is very important for the safety operation of a nuclear power plant to research the thermal stratification and temperature fluctuation phenomena which are caused by the cold water injecting into the branch pipe connected the emergency core cooling system (ECCS) with the main pipe. The thermal stratification phenomena in the ECCS branch pipe was simulated using the CFD code CFX with k-ε model. The numerical results are in good agreement with the experimental ones. By changing the flow rate of the leaked cold water and the structure of the branch pipe， the inherent relations between these parameters and the thermal stratification phenomena were further researched. Therefore， the main cause and some characteristics of the phenomena were obtained.