Abstract: The modal analysis method studying the elastic support rotating mirror for ultra-high speed photography was presented. With the finite element technology，the first and second modes of the three-faced aluminum alloy mirror were simulated under the stiff and elastic bearings condition. The simulation results show that the only two resonance points are in the speed ranges of 2.5×105 to 2.6×105 r/min and 2.6×105 to 2.7×105 r/min respectively, both resonances are first order flexure vibrations. The vibration of the first order modal occurs in vertical plane and that of the second occurs in level plane. The simulation results under the elastic supporting hypothesis are more close to that of the experimental results.