Abstract In magnetic bearings, the shaft is suspended by magnetic force instead of mechanical contact. Magnetic bearing can be divided into two categories, passive magnetic bearing that uses a permanent magnet and does not need a controller, and active magnetic bearing that can control the force of magnetism (Spring and damping coefficient). The rotor is in the form of an elastically flexible shaft made of steel and a rigid disk in the middle, which is called a Jeffcut rotor. In this research, the aim is to investigate the influence of nonlinear derivative proportional controller parameters on the dynamic behavior of the rotor with active magnetic bearings. A combination of both linear and non-linear derivative proportional controllers is proposed to control the system dynamics. The four-pole active magnetic bearing system is used as an active actuator through which controllable magnetic forces are applied to restrain the desired system. In order to maintain the geometrical center of the rotor at the origin of the coordinates in the center of the bearings, a current in excess of the dynamic control current of the system, called bias current i_0, was generated in the electromagnets to maintain the weight of the rotor in the y direction. The magnetic force produced by the magnetic poles is a non-linear function of the currents passing through the coils and the air gap between the poles and the axis. Also, the non-linear behavior of the system in the presence of forces created by active magnetic bearings was investigated by modeling the system as a flexible shaft, which also considered the centrifugal force and the weight force in the non-linear differential equations of motion. The nonlinear differential equations of motion in the y direction were calculated by the method of multiple scales using Maple software. The findings showed that in the range of 0.1