As for active magnetic bearing, ordinary electromagnetic force formula based on air gap well-distributed. However, radial active magnetic bearing always face the situation that the rotor is eccentric and the air gap doesn’t well-distributed. This paper gives the calculation method of the electromagnetic force at that situation through calculating the air gap and using integrative approach. Then analysis the influence factors---eccentric rotor distance from center and eccentric angle to electromagnetic force. The result can provide the basis for suspension characteristic of active magnetic bearing.