To research the influence of supply pressure fluctuation on the transient characteristics of gas bearing-rotor system,it is taken that the numerical simulation of fluid-structure interaction combine with experiment validation to study the rule of the rotor lower end offset and axis trajectory under no-load and loading conditions with different rotational speeds and the supply pressure reduction.The results show that supply pressure reduction on a slope causes the phenomenon that the distance that rotor lower end deviates from the ideal trajectory varies in a wavy manner,and the axis track repeatability of power frequency becomes worse.Before and after the supply pressure reduction,the offset difference increases with the increment of loading force or speed and during loading,it is significantly higher than that of the no-load state.