A realization of the multi-frequency technique for Kelvin Probe Force Microscopy (KPFM) is studied. In the multi-frequency KPFM, the first eigenmode of the cantilever probe is used for topographic imaging, while the second one is resonantly excited for surface potential. Thus, an additional feedback is needed to adjust the probe automatically. An analog feedback controller with bandwidth of about 5 kHz is designed based on the principle of conventional Proportional-integral Controller (PIC). Employing the controller, a novel multi-frequency KPFM is developed, whose sensitivity for surface potential measurement is demonstrated to be better than 5 mV. Topography and surface potential distribution for a kind of dielectric film with the injection of charge are imaged with the multi-frequency KPFM, suggesting its application in the characterization of electrical material and devices.