An underwater detection device adapted for sweeping a detection area between two transducer arrays (2,7) at a first and a second position, respectively, and detecting a detection object at a position between said first and second position, comprising: the first transducer array at the first position away from a position immediately above a detection object, said first transducer array adapted for • analyzing acoustic propagation properties of a sound wave received by a predetermined calibration sound source in the water; • transmitting a scanning sound phase-conjugate wave in a time domain in response to active phase conjugation, which is equivalent to time reversal processing, based on said acoustic propagation properties; • converging the scanning sound wave at a moving position, termed pseudo sound source, on the detection area detection object in the detection area, thus sweeping the detection area; the second transducer array comprising a monitoring unit adapted for • receiving the sound wave transmitted by the first transducer array and reflected at the detection object; • analyzing a waveform of the sound wave received by the second transducer array; • performing passive phase conjugation on the received sound wave; • detecting, based on said analysing and passive phase conjugation, the detection object. the first transducer array at the first position away from a position immediately above a detection object, said first transducer array adapted for • analyzing acoustic propagation properties of a sound wave received by a predetermined calibration sound source in the water; • transmitting a scanning sound phase-conjugate wave in a time domain in response to active phase conjugation, which is equivalent to time reversal processing, based on said acoustic propagation properties; • converging the scanning sound wave at a moving position, termed pseudo sound source, on the detection area detection object in the detection area, thus sweeping the detection area; • analyzing acoustic propagation properties of a sound wave received by a predetermined calibration sound source in the water; • transmitting a scanning sound phase-conjugate wave in a time domain in response to active phase conjugation, which is equivalent to time reversal processing, based on said acoustic propagation properties; • converging the scanning sound wave at a moving position, termed pseudo sound source, on the detection area detection object in the detection area, thus sweeping the detection area; the second transducer array comprising a monitoring unit adapted for • receiving the sound wave transmitted by the first transducer array and reflected at the detection object; • analyzing a waveform of the sound wave received by the second transducer array; • performing passive phase conjugation on the received sound wave; • detecting, based on said analysing and passive phase conjugation, the detection object. • receiving the sound wave transmitted by the first transducer array and reflected at the detection object; • analyzing a waveform of the sound wave received by the second transducer array; • performing passive phase conjugation on the received sound wave; • detecting, based on said analysing and passive phase conjugation, the detection object.