In order to reveal the evolution of low pressure flow field due to the vortex precession effect,then provide theoretical foundation for selecting the pressure sensor and its install location,detailed 3D unsteady simulation was carried out for a DN50 swirl flowmeter.The experiment results show that:the steadyunsteady two steps numerical method can accurately capture the pressure fluctuation frequency,the meter factor obtained by numerical simulation is agree well with the experimental value.The numerical results completely show the pressure fluctuation along the axial direction and circumferential direction under the vortex precession effect.Selecting rule for the sensitivity,size and install location of the pressure senor were given.