Time-dependent characters of the three-photon-coupling process of OPO has been calculated and the effects of a series of parameters such as pump intensity, time delay, crystal length, cavity length and output coupling coefficient have been analyzed. Calculation results presaged the existence of “relaxing oscillation” in travelling-wave OPO. By using a new kind of uncoated ring cavity,the tuning range from 0.41 μm to 2.4 μm and energy conversion efficiency of 22% have been obtained experimentally. The consistency between theory and experiments is demonstrated.