1. Population growth has entered a bottleneck period. In the past few decades, the process of urbanization has advanced rapidly, but the degree of urbanization has reached a high level. Population growth began to be limited by factors such as environment and resources, and the growth rate of urban population began to slow down.

2. The number of young adults has declined. Due to China's single two-child policy, the number of people born is decreasing. The young and middle-aged population is one of the main sources of urban population growth, and the decline in the number will also affect the growth rate of urban population.

3. Economic development has slowed down. Urban population growth is closely related to economic development. If the economic development slows down, it will affect the speed of population flowing into cities, leading to a slowdown in population growth.

Generally speaking, the cliff-like slowdown of population growth in many cities reflects the new situation of China's economic development and social change, which needs to be dealt with by formulating scientific urbanization planning and population policy.