The invention discloses a coal sample ultrasonic dominant frequency detecting device and method under the influence of three factors. A high pressure gas bottle is arranged on the external side of a box; the high pressure gas bottle is communicated with the interior of the box through a pressure increasing/reducing valve; a coal sample placing table is fixed on the bottom of the box, the top, which is corresponding to the coal sample placing table, of the box is provided with a through hole, the through hole is provided with a sealing piston that is matched with the through hole; an ultrasonic wave excitation probe and a ultrasonic wave receiving probe are symmetrically arranged in the box; the ultrasonic wave excitation probe and the ultrasonic wave receiving probe are individually connected to the inner lateral wall of the box through a spring support rod; and an ultrasonic wave detector is electrically connected to the ultrasonic wave excitation probe and the ultrasonic wave receiving probe through a connection wire. The provided device can simulate the actual situation of underground coals so as to obtain the responding characteristics of dominant frequency under the coupled three factors namely crustal stress, gas pressure, and physical mechanical properties of coal, and thus basic technical guide can be provided for preventing outburst of gas and coal.