Abstract: Analysis of the instantaneous angular speed （IAS） has been proven to effectively detect bearing faults. To investigate the generation mechanism of IAS signal double-impulses phenomenon in ball bearings， a dynamic model is proposed in this paper based on the coupling characteristics between the normal， tangential forces and the time-varying impact excitation， in which the influences of the rolling resistance and the sharp edges of local defect on the dynamic behavior are considered. In this model， the elastic quarter-space method is adopted to calculate the additional deformation induced by the local spalled edges and its corresponding time-varying displacement excitation model is deduced. Additional contact forces and time-varying displacement excitations are considered when steel ball enter into and exit from the defect. The disturbances of normal and tangential forces in the bearing under sharp-edge excitations of local fault are studied， and the time-varying tangential force excitation model is also deduced. The nonlinear dynamic equation sets are solved using the Runge-Kutta numerical integration method. Comparisons between the simulation results and the actual measured results of outer race local fault bearings show the established dynamic model can effectively reveal the mechanism of double-impulses phenomenon based on IAS signals.