Family income largely affects fertility behaviors under the universal twochild policy. The Fifth Plenary Session of the 19th CPC Central Committee proposed to reduce the cost of fertility, parenting and education. Using CFPS data collected before and after the recent family planning policy change, this research develops a twochild family income conversion sub matrix decomposition method, and empirically analyzes the characteristics of two-child family income mobility. A binomial logistic regression model is established to analyze the factors influencing the income mobility of twochild families. The results show that: (1)During the relaxation of fertility policy, the absolute income of two-child families has moved downward, and the speed of movement has increased. The possibility of income rank downward mobility has also increased.(2)The impact of having two children on income mobility differs across families in different income groups. Middleincome groups are the most affected while the highincome group is the least affected.(3)In addition to the individual and households' characteristics, the imbalance of economic development and urbanrural differences are still the important macro factors affecting the income mobility of two-child families in China.