Abstract: The dynamic stability of a rotating electrorheological(ER) sandwich beam with periodically varying velocity is studied by usingthe method of multiple scales. Assuming the angular velocity of the beam isgiven as a harmonic function of time, the rotating sandwich beam is regardedas a parametrically excited system. The effects of structure parameters andworking condition on the instability boundaries in parametric resonance ofthe rotating beam are investigated. Numerical results show that thevibration characteristic and dynamic stability of the rotating ER sandwichbeam can be adjusted when it subjected to an electric field, and the ERmaterial layer can be used to improve the dynamic stability of the rotatingflexible beams.