Complex systems exist everywhere,but up till now people have not yet familiarized themselves with the internal structures and characteristics. The present paper studies the complex systems from the viewpoints of system dynamics. It presents a definition of the complex system and discuses the general nature of complex systems. Complex systems are counterintuitive and insensitive to changes in many parameters. Socioeconomic systems tend toward low performance, frequently reacting to a policy change in the short run in a way opposite to how they react in the long run, and sometimes they resist policy changes stubbornly. The present paper also lays due emphasis on some important viewpoints of system dynamics on socio-economic and managerial systms.