Abstract: The root cause of the high on-line vibration measurement of the nuclear power half-speed turbo-generator is studied,and the result shows that the first natural frequency of the sensor support setting isclose to the forth times harmonic wave of the generator speed frequency,that is the root reason of the high on-line vibration.This paper uses the three-dimension modeling software to make fixture mode of the sensor support setting,and utilize finite element analysis（FEA） software to analyze and calculate its frequncy characteristics.Two schemes are offered by adding a mass block or increasing the stiffness of the sensor support setting,that change the natural frequency.So the resonance is avoided,and the authenticity and effectiveness of the on-line vibration is ensured.