A method for controlling the optical cavity length of ring laser is presented. The control of optical cavity length of ring laser is realized by controling the refractive index of dry air in capillary of ring laser by using of piezoelectric ceramics. The method has few influences on performance of ring laser. It has advantages of high speed of response,high sensitivity,and low power consumption. It can be applied to frequency stabilizing systems of ring laser with reflector and ring laser with total reflection prism. This method has important purposes in laser gyro and laser measure.