Abstract: Firstly,the reasons for the decreasing of the reheat steam temperature are analyzed.Secondly,the methods to calculate the effect of the changing of reheat steam temperature on the relative internal efficiency and ideal cycle thermal efficiency of the nuclear steam turbine are given,and the independence of relative internal efficiency and ideal cycle thermal efficiency of nuclear steam turbine is analyzed.Finally,the method to calculate the effect of the change of reheat steam temperature on the heat rate is given.The calculation results,by taking a 900 MW nuclear steam turbine as an example,show that when the reheat steam temperature decreases per 10 centigrade,the ideal cycle thermal efficiency decreases 0.0423%,the relative internal efficiency decreases 0.078%,and the heat rate of the turbine increases 0.1203%.