The motorized spindle is a spindle unit that integrates the rotating shaft and the rotor of the motor, and its structure is complicated. Due to machining or assembly errors, the rotor of the motorized spindle has a certain amount of eccentricity. In order to explore the vibration characteristics of the rotor caused by the eccentricity of the motorized spindle, the rotor eccentric model of the motorized spindle was established. The unbalanced magnetic pull (UMP) caused by the eccentricity was calculated with Maxwell stress tensor method. The UMP analytical equation was substituted into the Jeffcott rotor model, and the rotor eccentric vibration equation was obtained. Taking the permanent magnet synchronous (PMS) motorized spindle of a certain grinding machine as an example, the UMP was analyzed with FEM, and the UMP characteristic acting on the rotor was obtained. It was found that the UMP is always pointing to the direction of the smallest air gap. The vibration responses of the rotor under the action of the mass eccentric centrifugal force and UMP were studied under different rotational speeds. The research results show that when the motorized spindle is operating at low speed, the UMP is the main source of rotor vibration. With the increasing of the rotational speed, the effect of mass eccentric centrifugal force on the rotor vibration is more obvious. The magnitude of the UMP remains unchanged, and its frequency increases with the rotational speed, while the effect on the rotor vibration is weakened. The vibration issue of the grinding motorized spindle developed by a machine tool factory in trial operation was tested. Through the spindle vibration spectrum analysis it is concluded that the tested spindle has static eccentricity, and the spindle axis trajectory was measured, which verifies the aforementioned analysis.