Abstract: In the generation Ⅳ (Gen-Ⅳ) nuclear reactor systems, liquid metal cooled fast reactors are considered to be one of the most promising reactor types because of the advantages of value-added fuel, high intrinsic safety, nuclear nonproliferation features, and closed-cycle nuclear fuel. Although liquid metal reactors have been developed for many years, they still face some thermal-hydraulic challenges, such as thermal striping, thermal stratification phenomena, core rheological vibration, and system safety analysis, etc., due to the complexity of the reactor’s internal structure and the specificity of the coolant’s physicochemical properties. This paper focuses on the thermal striping phenomenon. Thermal striping is an important issue in liquid metal reactors, which brings great challenges to the safe operation of the reactor due to the incomplete mixing of the coolant flowing from the reactor core assembly leading to cyclic thermal stresses in the solid components, causing cracks to develop on the surface of the components. Many high cycle thermal fatigue failures of solid materials caused by thermal striping have been reported, such as cracks in the wall of sodium pump vessel of Phenix fast reactor in France, cracks in the control rod guide tube of PFR prototype sodium-cooled fast reactor in the United Kingdom, and main cold trap failures of BN-600 SFR in Russia. In this paper, the effects of the velocity ratios between the hot and cold fluids on the thermal striping characteristics were investigated by using the large eddy simulation (LES) method with the parallel five-jet model. The results show that with the increase of the velocity ratio, the average temperature decreases, the initial mixing height increases, but the mixing process of hot and cold fluids is delayed. The average temperature gradient of the center cold nozzle is smaller than that of the outer cold nozzle, but the temperature to reach thermal equilibrium is higher. With the increase of the axial height, the average temperature decreases, reaching the minimum at z=400 mm, where the mean temperature curve transitions from double-peak to single-peak, indicating that mixing of hot and cold fluids has been completed. With the increase of velocity ratio, the average amplitude of the temperature fluctuation decreases and the maximum dominant frequency tends to increase. When r＜1.0, the frequency distribution is within 30 Hz, and when r≥1.0, the frequency distribution is within 40 Hz. This work provides valuable insights into understanding the flow and heat transfer behavior of liquid metals, and provides important theoretical support for the optimal design and safe operation of upper chambers of liquid metal reactors.