The rotor inter-turn short circuit fault is one of the main electrical faults of turbo-generators. It can cause the unbalanced electromagnetic force to act on the rotor, leading to rotor vibration and even greatly affecting the security of the generator and power system. The accurate calculation of the electromagnetic force is significant for the stability of the generator and power system. By establishing the generator finite element model, the operation of the generator under a rotor winding short-circuit fault is simulated. The changes of magnetic field lines and air-gap field density are analyzed for the impacts of short position, short circuit number of turns and excitation current on the electromagnetic force. The equivalent magnetic flux method and the magnetomotive force superposition method are compared and come up with some improvements for the equivalent magnetic flux method. The calculation results of the unbalanced electromagnetic force by the two methods are compared with the results of the finite element method, and reasons for the difference are pointed out.