When executing operations in underwater space, the traditional underwater mother robot has some limitations, such as loud noise, large size and poor concealment, and the movement mode of the son microrobot usually uses the way of tail swing, fin oscillating water and propeller impetus, which causes it hard to scale down and lack of stability. In order to overcome these shortages, a novel biomimetic underwater mother-son robot system is designed. In this system, the spherical underwater mother robot uses the water-jet as the actuator to reduce noise and increase concealment, which can provide the energy and signal for the microrobot. Based on the principle of bionics, the microrobot is designed according to the salps, which has the advantages of high sports transparency, high concelment, and high stability. The calculation model of the actuators of the mother-son robot are established respectively, and the rotating models of the son microrobot is also established. Finally, the prototype of the mother-son robot is made, and the underwater motion tests of the robot are carried out to verify the effectiveness of the designed mother-son robotic system.