Naval battlefield is one of the important battlefields in military confrontation. The naval battle space is involved with air, water, underwater and coastal land, etc., and combat resources are enormous in quantity and are diverse from each other in capability. With the continuous development of unmanned system technology, the cross-domain unmanned swarm will become an important force in future maritime warfare. Taking the maritime cross-domain unmanned swarm as a research object, firstly, the paper combs out the related concepts and evolution process of cross-domain operations, and defines the connotation of cross-domain unmanned swarm. And then the development trend and current situation of the US military single-domain unmanned swarm project are described, and the main contents of the recent cross-domain unmanned swarm exercise are analyzed. Finally, the existing research results in the key technologies for the unmanned swarms are refined and summarized, and the challenges facing the development of cross-domain unmanned swarms are pointed out. Simultaneously, the future development trend of cross-domain unmanned swarms is given.