In order to solve the problem of efficient trajectory control of centimeter-scale robots with small size, a small jellyfish robot (JRT) is designed and developed, which is composed of an air chamber, a center of gravity adjustment device and a jellyfish-inspired thruster with a size of only ⌀6.8 cm × 5.3 cm. Its trajectory can be effectively controlled underwater through the coordination of the center of gravity adjustment device and jellyfish-inspired thruster. Underwater experiments verify the function of JRT swimming and hovering in the vertical direction, horizontal directional swimming, and horizontal ejection. By analyzing the force balance relationship and motion state data of JRT swimming in the vertical and horizontal directions, a trajectory control scheme is proposed. The speed of horizontal directional swimming can reach 1.7 cm/s, and the trajectory deviation is 0—3.4 cm, which shows that the JRT has a good trajectory control ability and is expected to be applied in underwater resource exploration, environmental monitoring, and other fields.