The invention discloses a submarine cable underwater synchronous paying-off underwater vehicle which is characterized by comprising a shipborne mechanical system, a synchronous pushing system, a pneumatic launching system, a calculation center control system and a dragging and underwater-moving system. The technologies of electric automatization, aerodynamics, mechatronics and single chip microcomputer control and the like are integrated, underwater operation can be achieved, and the underwater vehicle is less affected by stormy waves; automatic detection and real-time paying-off can be achieved, the positioning precision is 0.5-3 m, the course accuracy is 0.01 degree, and a cable line corridor can be accurately controlled; cable laying is high in speed and can be controlled in real time, and the highest cable laying speed can reach 50 KM/h; the automation degree is high, and the number of construction staff is greatly decreased; the integration degree is high, and the whole process can be monitored; construction obstacles for the cable line corridor do not need to be cleared away, so that the environment is protected; all the systems can be recycled, and secondary investment is low; the systems operate synchronously, and the tension force of submarine cable laying is reduced; the line first-aid repair capability is high, and the submarine cable underwater synchronous paying-off underwater vehicle is suitable for line reconstruction work after seaquakes; and the structure is reasonable, and the performance is reliable.