Abstract: Mechanical seal is a critical foundational component of turbopump in liquid rocket engines, the high/ultra-high speed is its typical operating characteristic, and the linear velocity is always far greater than 25 m/s and some even exceed 100 m/s. Under the high/ultra-high-speed conditions, the lubricating fluid generates a lot of friction heat due to the strong shear effect of rotor end face. Additionally, the fluid in sealed chamber also generates the stirred heat severely because of the high/ultra-high-speed stirring motion of rotor. The friction heat and stirred heat led to a severe temperature rise in seals. The liquid film vaporized due to the high temperature rise, leading to a decrease in sealing stability. The sealing end faces intermittently oscillated and separated due to the film vaporization, leading to the collision and wear, and causing the fracture of rings and sealing failure in severe cases. Moreover, the strong vaporization of fluid films led to an excessive wear of sealing end faces due to the dry friction, and results in a further increased in sealing temperature due to the solid friction heat. Besides, the thermal deformation of sealing end faces increased the above risks of seal failure. In view of the previous literatures focused only on the viscous friction heat in mechanical seals, an integrated three-dimensional thermohydrodynamic numerical model consisting of the fluid film, fluid in sealed chamber and sealing rings was developed to investigate the stirring flow characteristics, stirred heat generation influencing mechanism and evaluate the influencing level. The comparative analysis and parameterized evaluation of flow, heat generation and heat transfer indicated that the stirring flow generated by high/ultra-high-speed shearing of the rotor impeded the cold fluid of sealed chamber flow into the sealing gap, a flow dead zone is generated at the groove root under the turbulent flow regime, which impedes the cooling of fluid film. Due to the high/ultra-high-speed shear of rotor, the turbulent fluid in sealed chamber generated a larger effective viscosity, radial velocity gradient, and turbulent dissipation rate, which were the main reasons of stirred heat generation. The temperature difference between the sealing rings and the fluid in sealed chamber was decreased by the stirred heat generation, suppressing the convective heat transfer level. The above phenomenon led to a significant increase in stirred heat temperature rise and a significant deterioration in sealing performance. For the high rotational speed and small sealed chamber size, the turbulence and stirred heat were most significant, and the stirred heat temperature rise was up to 36 K, accounting for 54% of the total, the load carrying-capacity deteriorates by 4 514 N and the leakage rate increased by 0.022 m³/h. This work could enrich the research on turbulence effect and stirred heat, and provided ideas for controlling the stirred heat temperature rise and guidance for the optimization design in high-speed mechanical seals.