The heart of family fertility decision is the determination of the nature of the birth, namely whether the children are demands or investments. Only to find the mystery of fertility nature, can find out the policy design of fertility release under the backdrop of "comprehensive two-child policy". Therefore, this paper judges the fertility natures such as the number of births, the number of survivals, the number of boys and the number of girls from the perspectives of family time allocation and income by the sampled data from 2013 to 2015. It arrives at the conclusions as follows:firstly, care for the elderly and age are the main variables affecting fertility, the lack of the own property rights of the housing and lodgings are the main factors inhibiting birth, and the role of family time configuration in reproductive decisions is stronger than the role of income; secondly, the stronger role of family time configuration in reproductive decisions than income factor leads to more obvious investment feature of fertility; thirdly, income, the strength of supporting the elderly and old-age security verify this view; fourthly, the nature of the fertility as the investment makes the women of childbearing age do a tradeoff between labor participation and family reproduction, and as women's income rises, the rate of returns on investment is lower and brings to lower fertility rates; fifthly, the sex differences in fertility mainly result from group differences of fertility, namely urban-rural differences and income differences.