Abstract: Based on the dynamic balancing of rigid rotors, an integral digital tracking filter method is put forward for fundamental frequency detection. The principle of this method is to calculate the amplitude and phase of fundamental frequency by numerical integration taking into consideration of the discreteness of the spectrum of periodic signals and the orthogonality of trigonometric functions. The amplitude and phase calculation errors caused by numerical integration are also given in this paper. With fundamental frequency detection simulation, the present method is successfully applied to the in-situ dynamic balancing of a rigid rotor. Results show that the method is easy to use with high precision of calculation. The amplitude calculation error is less than the amplitude by two orders of magnitude, and the phase calculation error meets the industrial demand.