Abstract: The main feedwater pump frequency-doubled vibration occurred in Tianwan NPP units 3 and 4 in the process of commissioning. The frequency-doubled is approximately equal to the three times frequency of the pump structure according to the result of modal test. The three times frequency of the pump structure is increased by reinforcing the pedestal of the main feedwater pump. According to the test result, the vibration of the bearing pedestal is reduced obviously. The reinforcement results in good effect.