Ultrafast doubly resonant optical parametric oscillators (DRO) are widely used in the generation of mid-infrared broadband optical combs and mid-infrared ultrashort pulses. Because the signal and idler lights both oscillate in the optical cavity, DROs exhibit many different features against singly resonant optical parametric oscillators (SRO). One typical feature is the cavity length detuning in degenerate DRO. The DRO would work in non-degenerate, near-degenerate, and totally degenerately states with cavity length detuning. In order to analyze the effect of cavity length detuning on the work state of degeneracy DRO, the cavity length detuning characteristics of DRO pumped with low dispersion femtosecond laser were systematically studied and summarized based on numerical simulation and usual pumping condition. The production reason of the correlation characteristic was analyzed in theory.