Provided is an ultrasonic detector, including a detection area and a cover body. The cover body comprises at least two cavities, multiple fluid inlets and multiple fluid outlets which are in communication in sequence from inside to outside. The multiple fluid inlets, the multiple fluid outlets and the cavities are in fluid communication to form a fluid channel. The fluid channel is configured to guide fluid to flow along at least one bent path, and the detection area is located in an innermost one of the cavities.