Abstract: The shape control of salt-cavern gas storage is the key to restrict the gas storage capacity and realize the stability of gas storage, and the key technology to control the cavity shape is to control the interface position of dissolution inhibitor. Existing control method is high in monitoring cost and cannot provide real-time measurement. In order to improve these shortcomings, an optical fiber interface monitoring method was proposed in this paper. In this method, a new type of optical fiber interface tester is used for measurement, and the distributed optical fiber testing technology is used and the cable in the optical cable is heated. Brine and cushion medium (generally diesel and nitrogen) are different in specific heat capacity, so heat exchange happens between the optical cable and the surrounding medium, which leads to temperature change. In this way, the position of the medium interface can be determined according to the temperature difference. This device is advantageous with simple operation, wide detection range, low monitoring cost and real-time measurement of downhole medium interface. The application of this technology can realize large-scale reverse-circulation solution mining of salt-cavern gas storage, increase solution mining speed and guarantee cavity safety.