Objective:To explore the mediating effect of postpartum depression in fathers on their parenting competence and breastfeeding support.Methods:From March to October 2022, convenience sampling was used to select 337 fathers of infants aged 4 to 8 weeks in Wuxi Women and Enfants Care Hospital as the research subject. A survey was conducted using the Edinburgh Postnatal Depression Scale (EPDS), Chinese version of Parenting Sense of Competence Scale (C-PSOC), and maternal spouse version Partner Breastfeeding Influence Scale (PBIS). Spearman correlation was used to explore the correlation between postpartum depression in fathers and their parenting competence and breastfeeding support, and the PROCESS plugin was used for mediation effect testing.Results:A total of 337 questionnaires were distributed and 337 were collected, including 314 valid questionnaires with an effective response rate of 93.18% (314/337). Among 314 infant fathers, the EPDS, C-PSOC, and maternal spouse version PBIS scores were 3.00 (2.00, 6.00), 66.00 (62.00, 71.00), and 83.00 (74.00, 90.00), respectively. Father's parenting competence was negatively correlated with father's postpartum depression ( r=-0.435, P<0.01), and positively correlated with father's breastfeeding support ( r=0.480, P<0.01). Father's postpartum depression was negatively correlated with father's breastfeeding support ( r=-0.423, P<0.01). The mediating effect analysis showed that the mediating effect of postpartum depression in fathers between parenting competence and breastfeeding support was 0.341, accounting for 39.51% of the total effect. Conclusions:Postpartum depression in fathers is a mediating variable between their parenting competence and their breastfeeding support. Medical and nursing workers should develop perinatal education programs for fathers, enhance their parenting competence, alleviate their negative emotions, and promote their breastfeeding support.