For estimating the effect of the medium pressure rotor cooling system in ultra-supercritical turbine on the main steam flow and cooling efficiency,the steam-thermo coupled numerical simulation method was used to study the medium pressure rotor steam cooling system,with the 600 MW ultra-supercritical turbine medium pressure casing as the computational model.The computational results show that the maximum and minimum temperatures of the blade root are decreased by 19 K and 95 K respectively,and the medium pressure rotor is cooled at the present cooling structure and initial parameters of the cool steam.The nonuniformity of temperature distribution of the rotation blade is increased by the cool steam,so the thermal stress is enhanced.Thus the important problem for the rotation blade to be cooled is the thermal stress verification.