With man as a link, the socil system, economic system and natural system are integrated into a complex system, the social-economic-natural complex ecosystem (SENCE), although they are different with each other. In this paper, the relationship between population and sustainable development of SENCE is studied. It is pointed out that population is the key factor for the sustainable development of SENCE,and that the goal of the sustainable development of SENCE can not be realized unless the population is coord...