The invention relates to a magnetic coupling steering device used underwater. According to the magnetic coupling steering device, frictional losses of a conventional motive seal steering mode are reduced, meanwhile, the water leakage risk caused by abrasion of a sealing ring is avoided, and the reliability of the device is improved. The magnetic coupling steering device used underwater comprises a rudder body, a driving device, a magnetic coupling device and rudder blades rotationally connected with the rudder body. The driving device is arranged on the rudder body. The magnetic coupling device comprises a mounting base, an input shaft, a driven magnetic rotor, an output piece and a driving magnetic rotor. The mounting base is arranged on the rudder body. The driving magnetic rotor and the driven magnetic rotor are rotationally connected with the mounting base. One end of the input shaft is fixedly connected with the driving magnetic rotor, and the other end of the input shaft extends to the outside of the mounting base and is connected with the driving device through a transmission unit. One end of the output piece is fixedly connected with the driven magnetic rotor, and the other end of the output piece extends to the outside of the mounting base and is connected with the rudder blades through a transmission rod.