The invention discloses an underwater object precision positioning system and a method. The system comprises a mother ship, a computer with a plurality of serial ports, a desk computer, an ultra short base line positioning system, a differential GPS (global positioning system), a compass, an ROV (remote-operated vehicle) system, a forward-looking sonar camera, a low-illuminance black and white video camera, an attitude indicator and a temperature-salinity depth profiling instrument. A shore-based transceiver, an ROV water surface system unit and the like of the ultra-short base line positioning system are carried by the mother ship, and underwater system units such as a transponder of the ultra-short base line positioning system, the forward-looking sonar camera and the like are carried by an ROV submersible vehicle, so that the underwater object precision positioning system is formed. By the aid of acoustic positioning between the shore-based transceiver and the transponder of the ultra-short base line positioning system and acoustic positioning of forward-looking sonar, the shortcoming that an existing underwater GPS positioning system only can position an object carried with anacoustic response device in a water area is overcome, and longitude and latitude coordinates of an optional unknown object in an optional water area can be positioned in a WGS (world geodetic system)84 ellipsoidal coordinate system in real time.