Quantum repeater is a basic building block for long-distance quantum communications via entanglement distribution. The high retrieval efficiency is an important parameter to realizing ensemble-based quantum repeater in practice. The Duan-Lukin-Cirac-Zoll (DLCZ) protocol based on the cold atomic ensemble and the linear optics can generate and store entanglement, which is regarded as one of the most potential schemes. The DLCZ protocol can probabilistically create a single photon and store an atomic spin-wave simultaneously, then the spin-wave can be transformed into the photon by applying a read beam. Combined with the collective enhancement effect of atomic ensemble and cavity-enhanced mechanism, one can enhance the interaction between photon and atom and improve the retrieval efficiency. Here, we demonstrate the effect of the optical depth (OD) on the retrieval efficiency of cavity-enhanced quantum memory based on the 87Rb cold atomic ensemble. In this work, the ring cavity is placed around the cold atomic ensemble. The cavity length is 3.2 m and the finesse is 13. The optical loss of all ring cavities is 21%, mainly consisting of the loss of optical elements (15%) and the loss of the cell (6%). In order to improve the retrieval efficiency, we need to ensure the triple-mode resonance of the read-out photon, write-out photon and locking beam by selecting the appropriate length of the cavity. We obtain the different OD by changing the detuning of cooling beam relative to the atomic resonant transition. We also measure the retrieval efficiency as a function of the OD and calculate the enhancement factor. The results show that the OD of the ensemble reaches the maximum 14 when the detuning of the coolingbeam relative to the atomic resonant transition is 18.5 MHz, and the retrieval efficiency of transforming the spin-wave to photon is 21%, the cavity-enhanced factor 1.6 folds, the intrinsic retrieval efficiency 43.2%.