The operation principle of an electromagnetic harmonic drive is analyzed and its speed ratio equation is given. The equations of the displacements of the flexible ring under magnetic forces and the equations of the output torque between the flexible ring and rigid one are deduced. Using these equations, the displacement distribution of the flexible ring is investigated, and the effects of the system parameters on output torque are discussed. The results show that the current intensity of the coils, the airgap, the iron core length, and the position of the rigid ring have effects on the output torque for the drive system. The current intensity can be taken as the main control parameter of the output torque.