The invention relates to a water treatment device and system and a water treatment method. The device comprises an ultrasonic transducer and a thin-layer treatment chamber, wherein the thin-layer treatment chamber is provided with a first cover plate, a second cover plate and a tapered and gradually-enlarged flow channel; the tapered and gradually-enlarged flow channel is provided with one or morevalves; liquid is remained between the first cover plate and the second cover plate; a radiation surface of the ultrasonic transducer is fixed on the first cover plate; the ultrasonic transducer is used for emitting ultrasonic waves; the thin-layer treatment chamber is used for receiving the ultrasonic waves and generating ultrasonic cavitation; the tapered and gradually-enlarged flow channel isused for generating hydraulic cavitation; the hydraulic cavitation and the ultrasonic cavitation are coupled to grenade coupling cavitation; the coupling cavitation is used for treating water. According to the water treatment device provided by the invention, the washing groove type ultrasonic transducer is adopted and the diameter of the radiation surface is greater than that of a piezoelectric sheet, so that the device has the advantages of low price, stability in working and strong adaptability; a large radiation area covers a coupling region and an external region with an independent ultrasonic effect, so that wastes of ultrasonic energy are avoided.