Labor supply is an important source of economic growth．In this paper，we use the factorization method to quantify the contribution of labour supply in China's economic growth by decomposing the labor supply factor into the population size，the proportion of working-age population，and the labor participation rate factors．We put them all into the classic Cobb-Douglas production function．Based on the theoretical model derivation，this paper analyzes the time series data from 1978 to 2015．The results show that labor supply has a significant contribution to China's economic growth in a long term．Its impact and contribution are less significant than the fixed assets，but much greater than the human cap- ital．The main contribution of China's economic growth comes from the input of material capital rather than the labor supply．All of the three variables representing the scale of labor supply have significant effects on economic growth in the same direction，but the magnitude of the effects varies．The order of the contribution and elasticity of the three factors to the total economic output are the total population，the proportion of the working-age population，and the labor participation rate．Finally， in recent years the decrease of the proportion of working-age population does suppress China's economic growth，but this negative impact is to some extent offset by the increase in the size of the total population．

The optimum population under the constraint of resources and environment，by which both the con- straints of resources and the environment are met and a certain standard of living and development goals are achieved，is an important basis for China’ s future population policy considerations．Based on the concept of optimum population and the analysis of its internal mechanism under the constraint of resources and environment，12 indexes are selected from the fields of natural resources，environmental and socioeconomic development．Using the possibility- satisfiability model，we separately estimate the optimum scale of China’ s population under a single factor and different combinations of indexes in both 2030 and 2050 as the target point time．The results show that the lowest value of China’ s optimum population under the constraints of major resources and environment is around 1157-1322 million in 2030 and 1465-1626 million in 2050，while the desired optimum population of China would be 986-1311 million in 2030 and 1188-1389 million in 2050． Water resource is always the most important factor in restricting China’ s population growth in the future．The relationship between population，resources and environment would be more strained by 2030，and then would tend to improve after 2050．