Rotor unbalance is the common fault of rotating machinery, it affects the safe and stable operation of machine. In order to reduce rotor unbalance online and without human intervention, the highquality online active balancing device is important. The balanced condition should be kept by simple and reliable means. An electromagnetic balancer with permanent magnet was proposed, the permanent magnets were used to accomplish the balancing condition maintaince. In balancer design, magneitc field distribution and flux density were main invaluation indicators. Based on structural design and magnet choice, the selflocking magnetic field FE model was built, and then the influences of magnet performance, size and air gap on magnetic field were studied, based on the results the reasonable parameters were chosen. This work provides the base for new electromagnetic balancer design, assessment and optimization.