This paper considers the coupling effects between the left and right handed circular1y polarized modes，and obtains fundamental formulas. Under the special conditions that the couplings. between the same handed palarized modes are the chief ones，we oftain an approximate analytic solution，quantitatively describing the locking phenomenon of second kind，which show，that every simple laser gyro lies in travelling wave state，while the differential beat frequency locks. The results coincide with our experimental data in recent years. We point out also the effective ways to weaken it. Besides，we oftain the positive terms of relative scale factor correction，which are the generalization of corresponding terms in simple laser gyro.