As a kind of vehicle capable of simultaneously accomplishing underwater and aerial operations,the unmanned underwater-aerial cross-domain vehicle has become a hot research topic nowadays. In this paper,several research results on underwater-aerial cross-domain vehicle are briefly introduced,and an introduction to prototypes of different design ideas is given. This type of vehicle has broad application prospects on scientific investigation or military,but its development process is also full of difficulties and challenges. The current technology is still unable to complete the prototype for actual operation tasks. Therefore,the various difficulties that may be encountered in the development of the cross-domain vehicle have been introduced.