Long-distance entanglement distribution is an important task for quantum communication, but it is difficult to achieve long-distance entanglement distribution owing to the loss of photons in optical fiber transmission. Quantum Repeater is a scheme to solve this problem. In this scheme, the long distance of entanglement distribution is divided into several small parts, the entanglement is firstly established at both ends of each part, then, the entanglement distance is expanded through the entanglement exchange of adjacent intervals parts, in order to achieve long distance entanglement distribution. Among them, the DLCZ protocol based on the cold atom ensemble and the linear optics which can generate and store entanglement, is regarded as one of the most potential schemes. In the process of DLCZ, retrieval efficiency is an important index of the quantum repeater, because it will influence each entanglement exchange operation between adjacent quantum repeater nodes. Generally, the retrieval efficiency is improved by optimizing the reading pulse, increasing the optical depth (OD) of the atomic ensemble and the cavity enhancement. The ring cavity constrains the light field to increase the intensity of the interaction between light and atoms, and effectively improve the retrieval efficiency of the quantum memory. In this work, atomic ensembles are placed in ring cavity. The cavity length is 3.3 m and the finesse is 13.5. The optical loss of all ring cavity is 21%, mainly including 15% loss of other optical elements and 6% loss of the cell. In order to increase the retrieval efficiency, we need to ensure the mode resonance of read-out photon, write-out photon and locking. The cavity needs inputs two beams of light from the path of read-out photon and the path of write-out photon in reverse. The two beams are locked at the same frequency as the write-out photon and the read-out photon respectively. The cavity length is adjusted by moving the cavity mirrors position through the translation frame, to make two light mode resonate. The AOM is inserted into the path of the locking to control the frequency of the locking. By adjusting the AOM to change the frequency of the locking, the locking can be coincident with the write-out and read-out cavity modes. Then, the three mode resonance can be achieved When the cavity mode resonates with the atomic line, it will lead to the splitting of the atomic formants and affect the enhancement effect of retrieval efficiency. In the experiment, the detuning of the read light will affect the frequency of the read-out photon, and further affect the detuning of the cavity mode with the resonance line of the atom. Thus, by increasing the detuning between the reading light and the atomic transition line, the frequency splitting between the two modes can be reduced, then enhance the retrieval efficiency. We research the relationship between the enhancement factor of the retrieval efficiency and the detuning amount of the reading light relative to the atomic resonance line. As the results are follows: when read detuning amount of 80 MHz light to enhance the ratio of 1.68 times, the nature retrieval efficiency is 45%.