Abstract: The thermoacoustic instability is one of the most difficult problems in the development of aerospace propulsion systems. The research on the thermoacoustic instability mechanism of premixed flame can be useful to realize and solve the thermoacoustic oscillation problems of the practical engines. The characteristics of the premixed flame excited by acoustic wave are investigated by the simultaneous acetone/CH₂O planar laser-induced fluorescence（PLIF） technique, and the variation of unburnt zone and preheating zone is acquired. It is shown that the effect of the acoustic wave on the flame is more obvious at lower frequency and higher Sound Pressure Level （SPL）. As the frequency decreases and SPL increases, the change of the flow structure gradually aggravates. It is also found that the morphology of unburned zone/preheating zone changes periodically with the acoustic wave phase. The edge of acetone PLIF images is extracted, and the dependence of the lifting height, diffusion area of the premixed gas on the acoustic wave frequency and the SPL is obtained. The evolution process of the flame morphology under typical acoustic frequency and pressure level is acquired by the simultaneous acetone/CH₂O PLIF images. The phenomenon and mechanism of combustion flameout under typical acoustic condition are analyzed.