Fertility rates have been decreasing both worldwide and in China. Although current policies have been aimed at raising the birth rate in China, their overall effects have been unclear. Therefore, exploration of fertility intention and related influencing factors is crucial. This study used a convergent parallel mixed methods combining descriptive and cross-sectional designs with a framework analysis. Convenience sampling was used to select mothers of childbearing age with two children (n = 603), living in Guangdong Province, China. Participants completed an online questionnaire investigating sociodemographic characteristics, as well as determinants and attitudes regarding the third-child fertility intention. Correlation coefficients and multivariate regression analyses were used to present quantitative findings. Eleven interviews were conducted, and a framework analysis method was used for data analysis. The results for the qualitative and quantitative study components were analyzed separately and were subsequently integrated through side-by-side comparison and joint display. Attitudes toward fertility intention were negative, and the rate of fertility intention was 10.4% in Guangdong Province. In the quantitative component, women's age, perceived value of having a third child, and attitudes were found to be factors promoting the intention to have a third child, whereas monthly family income, grandparents' health status, and policy support were found to be factors hindering the intention to have a third child. Four major themes emerged regarding the perceptions and experiences regarding fertility intention in the qualitative component. After integrated analysis, younger age, knowledge regarding policy support, and access to childcare support were found to be factors significantly affecting the intentions of mothers with two children regarding having a third child. Fertility issues require continued attention. Our findings provide a reference for optimizing existing policies to increase fertility intentions. Providing policy support, such as public childcare support, flexible working hours for mothers, affordable childcare, and greater psychological support, would enable mothers to improve their parenting. In addition, increasing the dissemination of knowledge regarding policies is necessary to improve the level of public understanding and promote fertility intention.