In view of the fact that the frequency spectrum components of a turbo-shaft engine are numerous and unclear for vibration source or modulation, when it tested on test bed, the vibration data is filtered by empirical mode decomposition (EMD). It is found that the low-frequency component contains more significant information from the original data. After further multi-sensor and multi-state measurement and time-frequency analysis, it is demonstrated that the frequency component is the subsynchronous frequency of the gas generator combined with the rotor, and the ratio between them is 0.443. Finally, through the elimination method and the oil interruption of the whole engine, it is determined that this component is the characteristic frequency of the externally squeeze oil whirl of front elastic support on the gas generator combined withthe rotor.