The system design in fiber cavity ring-down spectroscopy (CRDS) and liquid sensing are studied in general. Firstly,the technique principle of CRDS is introduced. Secondly,the typical fiber ring-down cavity structures are classified. Thirdly,the design methods of CRDS system are proposed and our recent progress of the liquid sensing study in application of CRDS technique is briefly introduced. Finally,the development of the CRDS technique is presented,and study idea and project suggestion are proposed.