Ellipticity of modes in resonator and magnetic fields are two necessary conditions for magnetic sensitivity of ring laser gyroscope. Reducing ellipticity of modes is one of approaches to reduce magnetic sensitivity of ring laser gyroscope. Based on theory of resonator by Jones matrix,the positional principle of establishing selfconsistent equation is given, the method of choosing modes which exists is given, a theory of calculation on ellipticity of modes is formed. The mechanism of the effect of reflectors′ parameters and angle of folding plane of ring laser on ellipticity of modes in resonator are analyzed by analytical method of numerical value. Two new approaches to reduce ellipticity of modes are advanced, they are achieved by collocating the reflection-inducedretardances of mirrors. The results of research have important significance for study of engineering on reducing magnetic sensitivity of ring laser gyroscope.