Abstract: If water level changes abruptly in specific operating conditions such as power plant's black start, ships are prone to be capsized or stranded. In light of these conditions, a portable onboard apparatus for pre-alarming the abrupt change of water level is developed. The apparatus consists of temperature testing circuit, ultrasonic transmitting and receiving circuit, data display circuit and alarming circuit, with singlechip as the control core. Using this apparatus, three values such as the freeboard height, inclination of ships, and the water level change rate in the surrounding water area can be monitored instantaneously. When water level changes abruptly and one of the three values or all values exceed safety limit, the pre-alarming apparatus will automatically alarm and feedback the variations of these values to the crew in time, and alert the crew to adjust in order to ensure the safety of docked ships. Compared with traditional early warning system, it has many advantages such as low cost, small size, light weight, and easy installation. Finally, it can be applied to pre-alarming the abrupt change and safety monitoring of docked or navigational ships in the surrounding water area.