Aiming at keeping rotor dynamics stability during industrial steam turbine operation, an analysis method and analysis steps were introduced to analysis a steam turbine rotor with overhung which was designed by Hangzhou Steam Turbine Corporation. The mathematic model of rotor synchronous orbit was analysed and the reasons for rotor instability were indicated. The maximum oil fi｜m temperature difference and rotor thermal bending unbalance were analysed, and the thermal bending unbalance was compared with an unbalance threshold which was calculated to check safety. A method to check steam turbine stability safety was introduced. The results demonstrate the thermal bending un- balance of the steam turbine rotor is less than the unbalance threshold and the turbine would keep stable in its operation speed range with no instability occuring.