Based on a brief analysis of the narrowly-defined demographic transition and the classical model,this paper argues that the proposed theory of the second demographic transition broadly interprets the internal motivation and mechanism of action of the demographic changes by drawing upon the demographic "intermediate variables",reflecting on the relationship between modern demographic transition and social changes at the macro,meso and micro levels.Demographic transition is a multi-dimensional dynamic historical process,involving a set of transitions on marriage,reproduction,family,health,population control etc.Demographic transition is not only demographic change,but more importantly,is its nested social change.The unprecedented close relationship between the two reflects the social changes and social needs.Over the past 30 years China has made its population be adapted to economic development through family planning.In the next 30 to 50 years or even longer,China should make economic and social development be adapted to the changes and needs of the population.The end of development is to improve population survival and enhance life quality.