Abstract: It is very important for the safety operation of a nuclear power plant to realize and prevent the thermal fatigue caused by temperature fluctuation. Based on a modified k-ε turbulence model, the temperature fluctuation phenomenon occurred as the cold water injecting into the hot-water branch pipe, which connected the emergency core cooling system (ECCS) with the main pipe, was simulated. The numerical results are in good agreement with the experimental ones, and further, the distribution of temperature fluctuating in the whole pipeline are obtained for better monitoring the thermal fatigue.