Abstract: Clean and efficient fire extinguishing technology has been a hot research topic in fire science. The research of acoustic extinguishing technology originates from the discovery that the different acoustic modes of combustion noise can lead to an unstable oscillation of the flame and local flame extinction. Recently, acoustic extinguishing technology has gradually entered the research field of scholars because it is clean and exhibits no secondary pollution. To study the fire extinguishing mechanism and analyze the specific control behavior of the acoustic wave on an unclosed flame, the flame shapes and the combustion characteristics of the pool flame with 3, 4, and 5 cm diameters under a 30–90 Hz acoustic force were analyzed. The experimental system includes a high-speed camera, signal generator, power amplifier, loudspeaker, and acoustic signal analysis device. The flame image analysis shows that the transverse sound wave intensifies the unsteady flow of the vortex, and the flame shape under an acoustic force could be divided into three types of state: intermittent, deflective, and stable. The numerical analysis of the flame geometry shows that the flame surface is highly twisted and wrinkled under the intermittent and deflective states with a higher fractal dimension. The frequency-domain signal analysis of the flame area, height, and width shows that the flame signal is very unstable in the intermittent state, and the peak frequency domain is concentrated in the range of 0–10 Hz. The acoustic frequency is always prominent in the frequency distribution of the flame width signal. Based on the relationship between the flame inclination angle and Richardson number, the form of the latter under the action of the acoustic wave was proposed. In the 50–70 Hz range, the response of the flame to the acoustic frequency was particularly considerable, and there may be a marginal effect when the acoustic frequency is higher or lower than the said range. The critical Ri_a⁻¹ of the intermittent and deflective states are 10.32 and 2.92, respectively.