Abstract: In the context of new-type urbanization, a thorough understanding of what impact county-to-city migration has on the child- bearing plan of rural migrants is important for the optimization of China's family planning policy and the realization of a moderate birth rate in China. This paper uses the data of national dynamic survey of Health and Family Planning of Migrant Population in 2016 to explore the effect that county-to-city migration has on the secomd-child birth of rural migrants and its underlying mechanism. The study shows that:(1) county-to-city migration has a significant negative effect on the second-child birth of rural migrants no matter whether they will live in city for a long time or permanently, and the effect on permanent settlers is more significant; (2) the interruption, selection and adaptation effects all significantly moderate the negative effect; (3) the interruption effect strengthens the negative effect by delaying migrants' first marriage/childbearing age and extending the birth interval; (4) the selection effect spreads the negative effect as rural migrants prefer stable jobs, leave their original birth place where birth rate remaims high, and improve housing finance options; and (5) the adaptation effect amplifies the negative effect as more urban jobs are taken by migrant family members, left-behind elders are no longer able to take care of their grandchildren in city, and urban pension insurance is available to rural migrants. Therefore, in order to achieve a balanced and sustainable population development, efforts should be made to enhance the inclusiveness of childbearing policies, construct a marriage and childbirth market that combines city and county together, improve the marriage and childbearing support system for new migrants, promote the synergistic implementation of the relevant economic and social policies, and create a favorable childbearing environment.

Key words: rural migrants, second child birth, county-to-city migration