The invention discloses an ultrasonic monitor device of net cage fish culture biomass evaluation. An ultrasonic transducer emits ultrasonic probe pulse signals to cultured fishes in a net cage and receives reflection ultrasonic pulse signals of the cultured fishes in the net cage; and the reflection ultrasonic pulse signals are processed through an ultrasonic transceiver and a signal processing device, therefore, the invention realizes real-time monitor to a water range in the net cage, reflects the condition of the cultured fishes in the net cage, carries out biomass evaluation, displays a treatment result through a display device and realizes the purpose of the invention.