This article first introduces the classification,characteristics,composition and application of ROV. The design concept of cascaded ROV is put forward based on practical problems,and the characteristics and composition of cascaded ROV are explained briefly. The key technologies and solutions that need to be solved in the design of cascaded ROV are discussed from six aspects,i.e. overall control,overall design,underwater deployment and recovery,communication between mother ship and cascaded ROV,redundant design and modular design, underwater acoustic and electromagnetic protection technology. Finally,the typical applications of cascaded ROV is illustrated with examples.