Abstract: The concepts of swarm intelligence are described, and its properties such as self-repairing,indirect communication and evolutionary learning are mentioned.Knowledge emergence in swarm intelligence is analyzed then.The essence of natural intelligence is discussed.By comparing with complex adaptive system,we regard the swarm system as a complex adaptive system.Several models and experiments are cited for more explicit demonstration.The concept of system intelligence is proposed based on summarizing the limitations of swarm intelligence.Finally,some conclusions about the significance and trends of swarm intelligence are given.