Abstract: In response to the operational characteristics of robots in extreme environments, the current state and future trends of swarm robotics based on simple robots are reviewed. The concept of simple robots and their key features are defined, and their advantages in extreme environments are outlined. The current research on simple swarm robots is summarized at three dimensions: individual characteristics, group interaction, and system emergence. The emergence of complex swarm intelligence from local interactions among simple robots is reviewed. Performance analysis methods for swarm robots are summarized from three perspectives: robot simulation, robotic experiment, and formal verification. Finally, the challenges and key scientific problems faced in the transition of swarm robotics from laboratory research to cooperative operation in extreme environments are identified, and future development directions are proposed.