Low frequency vibration has occurred in the turbine pump of one type of liquid-propellant rocket engine,which threatens the safety and reliability of the rockets and their engines seriously.To figure out the mechanism of the low frequency vibration faults,in this paper,a dynamical model of a immersion turbine pump rotor system was established for the consideration of the vibration caused by small gap annual flow excitation which properly happened in immersion turbine pump rotor.The Newmark directly integral method was used to simulate the vibration characteristics of turbine pump,and an experiment was also carried out to investigated this problem.Results showed that,the small gap annual flow excitation could induce large amplitude low frequency vibration on the turbine pump rotor bearing system.The amplitude and frequency of low frequency vibration had relation with the parameters of the turbine pump clearance structure,such as the clearance ratio,length-diameter ratio and the viscosity of the fluid medium.In order to avoid the vibration induced by the small gap annual flow excitation,sealing effect should be improved and length-diameter ratio should be decreased. Low frequency vibration has occurred in the turbine pump of one type of liquid-propellant rocket engine, which threatens the safety and reliability of the rockets and their engines seriously. Figure out the mechanism of the low frequency vibration faults, in this paper, a dynamical model of a immersion turbine pump rotor system was established for the consideration of the vibration caused by small gap annual flow excitation which normally happened in immersion turbine pump rotor.The Newmark directly integral method was used to simulate the vibration characteristics of turbine pump, and an experiment was also carried out to investigate this problem. Results showed that, the small gap annual flow excitation could induce large amplitude low frequency vibration on the turbine pump rotor bearing system. amplitude and frequency of low frequency vibration had relation with the parameters of the turbine pump clearance structure, such as the clearance ratio, length-diameter ratio and the viscosit y of the fluid medium. order to avoid the vibration induced by the small gap annual flow excitation, sealing effect should be improved and length-diameter ratio should be decreased.