The invention provides a small-size underwater magnetic coupling propeller device which comprises a motor assembly, a magnetic coupling coupler assembly and a propeller assembly. The magnetic coupling coupler assembly comprises a spacer bush, a supporting ring, an inner rotor, an outer rotor, magnets, a rolling bearing and an elastic retaining ring, the supporting ring is installed in the spacer bush, the inner rotor is installed on the spacer bush and the supporting ring, a groove is arranged on the outer side of the inner rotor, a first magnet is installed in the groove, the outer rotor is installed on the spacer bush and located outside the spacer bush, the outer rotor is provided with a groove, a second magnet is installed in the groove, the left side of an inner ring of the rolling bearing leans against the spacer bush, the right side of the inner ring of the roller bearing leans against the elastic retaining ring, the left side of an outer ring of the rolling bearing leans against the outer rotor, the right side of the outer ring of the rolling bearing leans against the propeller assembly, the outer rotor is connected with the propeller assembly, and an output end of the motor assembly extends into the inner rotor and drives the inner rotor to rotate through a key. The small-size underwater magnetic coupling propeller device is small in size, light, good in tightness, easy to process and applicable to micro underwater robots.