Taking the S-type metal welded bellows as the research object, a coupling model of mechanical seal was established aiming at the problem of vibration failure of bellows induced by fluid and prestress coupling excitation in the seal cavity; the vibration mode, natural frequency and equivalent strain of bellows under different working conditions were obtained by comparing and analyzing the influence of fluid and prestress on the mode of bellows. The results show that the modal of bellows changes obviously under the action of external flow field and the prestress hardly affects the natural frequency of bellows; when the flow field pressure is constant, the equivalent strain of bellows decreases with the increase of speed and prestress; the maximum equivalent strain and stress of bellows increase with the increase of inlet pressure of fluids.The conclusion provides a theoretical basis for the failure mechanism of mechanical seals and the optimization design of bellows.