The invention belongs to the technical field of ships and particularly relates to a shipborne portable water level mutation warning instrument which comprises a larboard ultrasonic sensor arranged on the larboard and a starboard ultrasonic sensor arranged on the larboard. The larboard ultrasonic sensor and the starboard ultrasonic sensor are connected with a single chip microcomputer. The single chip microcomputer is connected with a display device through a decoder and further connected with a timer and a buzzer alarm. The larboard ultrasonic sensor and the starboard ultrasonic sensor are provided with buffer type installation mechanisms and connected with a ship body through the buffer type installation mechanisms. The shipborne portable water level mutation warning instrument has the advantages that the ship freeboard height, the transverse gradient and the water level variability of a water area where the ship is located can be monitored in real time, when the water level changes suddenly, a sailor can adjust the posture of the ship in time according to the data displayed by the warning instrument and a warning sign given by the warning instrument, and accordingly the ship can be protected against risks such as capsizing and stranding.