Based on investigations of resonance characteristics in Brillouin fiber ring resonator, a frequency stability system is developed, in which microprocessor is adopted as the central controller and piezolectric ceramics (PZT) as the frequency regulator. In order to eliminate effects of external factors, the idea of “numerical average filter” is adopted, which has highly improved the accuracy of frequency-discrimination. Besides, in order to make full use of the high speed characteristic of the frequency stability system designed in this paper, the control idea of “equal step, small step and quick control” is adopted. Using this control idea can improve the control accuracy and still guarantee the tracking velocity, and the system will not be easy to lose locking because the control idea rejects control vibration. The measuring frequency time is about 500 μs, and frequency accuracy is about 0.5 MHz, the locking time is 30 min.