Abstract: Through full-scale engine vibration investigations in recent years we found that in a number of the engines tested there existed the phenomenon of 1/2 order sub-harmonic vibration（the half-frequency whirl）,which often became the major cause for excessive vibration level in test.This is a phenomenon of non-linear self-excited vibration.If we find out its source and inhibit or eliminate it,the mechanical performance of such a type of engine will certainly be improved.Based on the characteristics of the nonsynchronous whirls as shown in engine vibration spectra and by taking into account the details of construction of the engine,we have analysed four possible major factors of self-excited vibration: the unbalanced torque force caused by circumferential variations of blade tip clearances)the aero-elastic effect of the radial clearances of labyrinth seal)the fric-tional force within the splined coupling)and the non-linear stiffness of the supports.This paper summarizes briefly the 1/2 order sub-harmonic vibration phenomena observed in engine test,and,taking the overhung rotor as the mathematical model,deduces its dynamic equations with considering the effect of the major factors mentioned above.This paper discusses the destabilizing factors in connection with a specific type of aircraft engine,and studies the dynamics of the overhung low-pressure turbine rotor under the combined action of these factors for the purpose of explaining the phenomena observed in the spectra of engine vibrations.Investigations show that the frictional force within the splined coupling is the most important factor of self-excited vibration,and the nonlinear stiffness of the back support is responsible for transforming the self-excited vibration into autotracking 1/2 order sub-harmonic whirl.The results obtained from numerical calculation and simulation on an analoque computer are favourable to explain the various kinds of non-synchronous whirls observed in test.This paper also studies the possibility of improving system stability by modification of labyrinth（comb-type）seal,and suggests some measures for inhibiting 1/2 order sub-harmonic whirl.