In recent years, bowing of steam turbine rotor under long time service occurs in several high-parameter units. Collected data show that the bending of the haywire rotor is increasing continuously, which results in excessive vibration in operation and even causes over-limit vibration during start-up. In order to suppress the vibration, balancing is utilized in field with the traditional approach that the balancing mass is placed in the section of the rotor close to the bearing. However, the balancing with the traditional approach could only reduce the vibration temporarily. In the long time scale, the bowing is still propagating or even gets worse after the balancing. To determine the cause of bowing and form optimal balancing approach, analysis is carried out in this work including: (i) fault cause and its treatment of bowing of steam turbine rotor under long time service is studied with elastic–plastic mechanics and creep mechanism taken in account; (ii) a case study was carried out, where the bowing process was simulated and validated with the field monitoring data; (iii) the phenomenon of the traditional balancing method was illustrated with rotordynamics analysis, where the influence of whirling is included. Based on the analysis, the cause of bowing is determined as uneven creep effect. And the balancing method would influence the whirling mode, which would worsen bowing in the traditional balancing method. Based on this conclusion, an optimized balancing method was developed to reduce the vibration and prevent bowing propagation simultaneously.