An overwater life-saving system using sonar for monitoring relates to an overwater life-saving system for preventing and avoiding the occurrence of drowning accidents in swimming pools and open water. The life-saving system utilizes sonar communication technology in water, telecommunication technology in air and computer data processing technology, a sound signal of call for help is emitted by a sound signal emitting device adorned at the wrist of a swimmer and then transmitted to a sound signal receiving device by in-water transmission, then a radio signal emitting device of a radio signal processing system is utilized for transmitting the sound signal for calling for help to a shore-based radio signal receiving device by in-air transmission, later the sound signal for calling for help is transmitted to a computer of a rescue station by cable, and finally the computer is utilized for analyzing and accurately positioning to find the place of a drowning man, so that the lifesavers and managers can save the drowning man in time, thus ensuring the life safety of swimmers and avoiding the occurrence of drowning death accidents.