Experimental modal analysis is done to the grinding motorized spindle under 36000r/min. The corresponding theory and experimental plan are introduced. The time domain waveform is gained and the maximum vibration velocity is worked out. Stochastic Subspace Identification (SSI) is applied to study dynamic characteristics of motorized spindle which is running idle, then modal parameters are extracted. The correctness of the experimental method is validated, which has certain referential importance.