In this paper, the vibration of a three screw pump was studied. Firstly the vibration response of the pump was tested. Then the FE model of the pump was constructed and the modals of it were calculated. The exactness of the result was confirmed by the testing modals and the reasonable of the FE model was verified at the same time. Finally, the force acted on rotors due to pressure was calculated by the mathematical method of surface integral. The force was loaded on the FE model and the force response was predicted, which represented the basic vibration of the screw pump. Based on the work above, the vibration spectrum of the screw pump was analyzed.