Abstract: In China, diesel is injected into the top of the cavity to ensure the side solution and control the cavity shape during solution mining of salt-cavern gas storage. If the diesel-brine interface is not positioned accurately or out of control in the process of solution mining, the cavity roof will be dissolved, the cavity shape will be out of control, the cavity volume will be lost, and solution penetration will occur at the production casing shoe. Thus, cavity stability will be destroyed. And consequently, the economic benefit and operation safety of the underground gas storage will be impacted directly and even the cavity will be abandoned. In China, neutron log is currently the most widely-used way to monitor the diesel-brine interface, but it is expensive with complex operation procedure and cannot meet the demand of continuous monitoring. In this paper, the optical fiber based diesel-brine interface monitoring technology and the corresponding monitoring tool (i.e., optical fiber based diesel-brine interface monitoring tool), which were developed by utilizing the differences of the specific heat capacity between diesel and brine in the cavity, were applied in 8 wells in Jintan Salt-Cavern Gas Storage, and the depth of diesel-brine interface was monitored continuously without stopping the production. Practical application indicates that the technology has many advantages, such as continuous monitoring, high precision, low cost and convenient operation. The technology has been widely applied in Jintan Salt-Cavern Gas Storage.