The present invention is to provide an underwater monitoring system for a fish farm using a variable frequency type scanning sonar, and a method thereof. The scanning sonar comprises a single transducer which transmits transmission ultrasonic waves； a transmission signal amplification unit which generates a transmission signal provided to the single transducer based on a complex modulation signal； a modulation unit which generates the complex modulation signal based on a resonance frequency and an upper limit frequency, and provides the complex modulation signal to the transmission signal amplification unit； a comparison reference signal generation unit which adaptively generates an ultrasonic echo signal obtained from the transmission ultrasonic waves received after being reflected from an outside object, and a comparison reference signal calculated by a reference signal based on the transmission signal； a comparison unit which generates an ultrasonic output signal by comparing the comparison reference signal with the received ultrasonic echo signal； and a signal processing control unit which calculates a distance from the outside object based on the ultrasonic output signal, and controls a long distance mode or a short distance mode based on the calculated distance. In the long distance mode, the transmission ultrasonic waves are transmitted through the resonance frequency. In the short distance mode, the transmission ultrasonic waves are transmitted through the upper limit frequency. Therefore, the underwater monitoring system and method for a fish farm can monitor a long distance and a short distance at the same time by using the scanning sonar including a single transducer in a variable frequency type.본 발명에 따르면, 송신 초음파를 송신하는 단일 트랜스 듀서； 복합 변조 신호에 기초하여 상기 단일 트랜스 듀서에 제공되는 송신 신호를 생성하는 송신 신호 증폭부； 공진 주파수 및 상한 주파수에 기초하여 상기 복합 변조 신호를 생성하고 상기 송신 신호 증폭부에 제공하는 변조부； 상기 송신 초음파가 외부 물체로부터 반사되어 수신된 초음파 반향 신호 및 상기 송신 신호에 기초하는 레퍼런스 신호에 의해 산출된 비교 기준 신호를 적응적으로 생성하는 비교 기준 신호 생성부； 상
