China's population will start to drop and age rapidly in the third decade of the 21st century. This negative population growth brings various challenges to China's future economic development. To sustain the economic growth and development, it is necessary to explore theoretical basis for the driving forces and new paths for further economic development. Reflecting on the limitations of existing theories of population and economic development and drawing on their enlightenments and metaphors, this research demonstrates how population size interacts with economic development. Further, this study proposes the concept and theoretical framework of population cushion, which can act as a new driving force of Chinas economic development in the context of negative population growth in the future.