To explore the impact of fertility support policy on fertility desire and its mechanisms, this study divides fertility support policy into pregnancy support policy and parenting support policy based on fertility behavior, and conductes empirical analysis using the special survey data of “Effectiveness evaluation of fertility policy and supporting measures” in Chengdu in 2021. The results show that there is a significant positive relationship between pregnancy support policy and fertility desire, while the relationship between parenting support policy and fertility intention is not supported by empirical evidence. In further exploring of the differential mechanisms of fertility cost perceptions and fertility preferences in the impact of pregnancy support policies on fertility desire, the childbearing age groups can be classified into four categories: “daring but not willing to give birth”, “not willing and not daring to give birth”, “daring and willing to give birth” and “willing but not daring to give birth”. Different policy directions are applicable to different types of childbearing age groups under different mechanisms. Therefore, constructing a long-term fertility support policy system requires the scientific identification of target groups and precise policies to improve policy effectiveness and to effectively enhance fertility levels.