Abstract: The temperature fluctuation phenomenon caused by the mixed coolant flows out of the core with different temperatures in upper plenum, may not only interfere with the temperature measuring in the outlet of the core, but also cause thermal fatigue of the solid structure in upper plenum. In this paper, the three-dimensional triple-jet flow was simulated by the software FLUENT with large eddy simulation (LES). The numerical results are in good agreement with the experimental ones. The results show that the mixing of fluids becomes more and more intense with the inlet velocity increasing. The maximum amplitude of temperature fluctuation decreases obviously, while its corresponding frequency increases in the case.