A method to deduce the electromagnetic force expressions of end windings based on the image theory, the Biot-Savart law and the superposition principle is proposed as the end structure frame of large capacity turbo-generator is weak and easily to occur accident. On the basis of a 600 MW home made large capacity turbo-generator, the results obtained by analyzing the same segment of each end bar that distributed along the circumference and the key points demonstrate that, the position of the tangential electromagnetic force maximum is regular at different moments, and the radial and tangential electromagnetic force are the same quantity, the radial electromagnetic force is bigger than tangential electromagnetic force in the whole, but the maximum radial electromagnetic force is less than the maximum tangential electromagnetic force. The simulation results provide some references for the fixture of the end structure and the prevention of the accident.