Small-size underwater drones swarm has the characteristics of small volume , low power consumption,swarm intelligence,etc,and can be used in ship troubleshooting,undersea security inspection,seabed mining and communication relay. Typical small-size underwater drones swarms,such as CoCoRo,M-AUEs and SmarmDiver have been used to mimic marine organism behavior,monitor marine environment or implement underwater searching. The technologies of multi-sensor carrying,data fusion,underwater communication, localization and swarm control of small-size underwater drones swarm should be improved further,and hence we should strengthen research and development in this domain.