The invention relates to the technical field of aluminum alloys, and further discloses an ultrasonic liquid injection device and method for the lithium ion battery. The device comprises a liquid injection box shell, the top of the liquid injection box shell communicates with a vacuum nitrogen pipeline, a conversion box is arranged in the liquid injection box shell, connecting rods which are evenlydistributed are fixedly connected to the top of the conversion box, and the top ends of the connecting rods are fixedly connected with the liquid injection box shell. According to the ultrasonic liquid injection device and method for a lithium ion battery, the vacuum nitrogen pipeline is arranged, the interior of the shell of the liquid injection box can be pumped to a required vacuum pressure value, by arranging the ultrasonic generation device, ultrasonic waves can be emitted into the liquid injection box shell, under the interference of a vacuum environment condition and ultrasonic waves,the surface contact angle between the electrolyte and materials such as positive and negative electrodes of the lithium ion battery and a diaphragm can be destroyed, the surface tension is reduced, the infiltration speed is increased, and the method is short in liquid injection time, convenient to operate, high in reliability, high in practicability and convenient for a user to use.