A shut-down accident occurred in a power plant because of turbine’s partial frequency operation. The main reasons of frequent fluctuations of steam temperature behind high and low pressure bypass in HP casing exhaust lie in high metal temperature, which is caused by the closed non-return valve and the heavy turbine load. Meanwhile, the harm to turbines caused by the removal of high pressure cylinder under large load is analyzed. Finally, some suggestions on logical configuration of partial frequency protection, desuperheating water of low pressure bypass and non-return valve are proposed in HP casing exhaust. The operation practice show that the logical configuration scheme could avoid accidents and be safe and reliable.