On the basis of the requirements of cavity adjustment completed by measurement ellipticity, an ellipticity measurement system with double beam paths was researched for adjusting the cavity of a ring gyro laser. The frequency response characteristics of a square ring resonator to the S line polarized light and P line polarized light were analyzed, and the S line polarized light was chosen to stabilize the frequency of a positive resonant cavity. By inputting the S polarized beam to improve the frequency stability accurately, a frequency stabilization system and a ellipticity measurement system with double beam paths were established. By utilizing the new frequency-stabilization system and measurement methods, the frequency stabilization of the system can be realized in 10 s and the stabilization accuracy is better than 1.8%. Furthermore, the fluctuation of ellipticity has been 0.65% by the proposed method, which reduces the effect of light fluctuation on the measurement accuracy. In conclusion, the proposed system meets the needs of ellipticity measurement.