The invention relates to an ultrasonic reactor based on a diffuse sound field, in particular to a device for treating liquid by utilizing an ultrasonic cavitation effect. The ultrasonic reactor comprises a tank body (1), an ultrasonic transducer (3) and a sound reflector (20), wherein the tank body (1) is used for containing liquid to be treated; the ultrasonic transducer (3) is installed in the tank body; and the sound reflector (20) is positioned on the wall surface of the tank body opposite to the radiating surface (24) of the ultrasonic transducer (3) and is provided with a sound reflecting surface which is not perpendicular to influent sound waves emitted by the ultrasonic transducer (3). The ultrasonic reactor carries out sound treatment to the liquid in the ultrasonic reactor by utilizing a uniform ultrasonic field to realize the uniform sound treatment of the ultrasonic reactor and improve the utilization ratio of sound energy in the ultrasonic reactor.