The optimal population in each period is different, and its value gradually increases with the development of society. However, its growth curve is not linear or S-shaped, but stepped, with different lengths, slow in time dimension and limited in scale dimension. The optimal population must be a scale suitable for social and economic development. At present, the total population has exceeded the optimal population12 ~1300 million (the population size in the fastest economic development period). Only when the socio-economic structure makes revolutionary progress can the optimal population reach a higher level. Some cities are overpopulated, and the rural population, especially the young and middle-aged population, is decreasing too fast. Compared with the population density of Asian and European countries, China is not a densely populated country. The population density of different continents is inconsistent, and there are geographical isolation restrictions, which is not convenient for comparison. For China, it is not the total population that restricts the growth of per capita income, but the excessive floating population and underemployment. If the country can retain the young people in rural areas, support the development of rural economy and change from "entering the city" to "urbanization in rural areas", the prosperity of the whole people will be realized. China's economic model and industrial structure are changing, the economic model is changing from foreign trade to domestic demand, and the first and second main industries are developing to the second and third main industries. The transformation of economic model needs not more consumers, but more consumption consciousness. To expand consumption, we must consider how to let the people take their money out of the bank for investment or trading, and of course, we must also solve the worries of the people, that is, childbirth, education, medical care and old-age care. Production automation can save labor force, so when the industrial structure is upgraded, the surplus labor force will enter the tertiary industry and promote its development, which requires the guidance and support of the government. The east and the west go hand in hand, the urban and rural areas develop in a balanced way, and the industrial structure is upgraded, which will surely make the optimal population size of by going up one flight of stairs. The reality is that the low fertility rate caused by family planning and the pressure of urban life has lasted for decades, and the total population will inevitably decrease in the middle of this century. This period is also a critical period of economic transformation. Only by encouraging childbearing and reversing the downward trend of population can population and economy develop in harmony. By then, the optimal population will reach1500 million, but it will take a hundred years.