To improve the efficiency of thermal power unit and reduce the pollutant emissions, the steam turbine unit in series of transformation on flow is changed, the quality and center position of mass of the rotor is changed, which leads to the change of rotor critical speed and a certain effects on the run. In this paper, using the experiment to measure the different wheel quality and position of the rotor critical speed, the results are compared with the theoretical calculation result analysis, and the sensitivity analysis methods is introduced to analyze the influence of the quality of wheel and position on changing of steam turbine rotor critical speed. Experimental study agrees well with the theoretical analysis: the rotor critical speed is decreased with the increasing of the wheel quality and is increased with the increasing of the wheel offset. The sensitivity coefficient of wheel offset on the critical speed is between 0.25 and 2.4, the greater the offset, the higher the sensitivity coefficient; the sensitivity coefficient of the quality on the critical speed is between -0.35 and -0.001, the greater the increasing ratio of the quality, the smaller the sensitivity coefficient; the influence of the offset on the critical speed is far greater than the influence of the quality, which is about 7~10 times. When the quality offset of the same wheel is greater than 40%, and the rotor critical speed changes obviously. In the same offset location, the increasing of the mass is less than 50%, and the rotor critical speed changes obviously. The method proposed can be used to solve the problem of the adjustment of the steam turbine rotor critical speed.