IntroductionThough evidence has revealed the beneficial effects of cognitive improvement interventions on breastfeeding, the effect of psychological interventions has rarely been studied. This study aims to test whether promoting a positive emotion intervention, ‘Three Good Things’ intervention, during the last trimester of pregnancy can enhance early colostrum secretion and breastfeeding behaviours by modulating the hormones associated with lactation (prolactin and insulin-like growth factor I). We will attempt to promote exclusive breastfeeding by using physiological behavioural measures.Methods and analysisThis study is designed as a randomised controlled trial conducted in the Women’s Hospital School of Medicine at Zhejiang University and the Wuyi First People’s Hospital. The participants will be randomly divided into two groups using stratified random grouping: the intervention group will receive ‘Three Good Things’ intervention, while the control group will write about three things that come to mind first. These interventions will be continued from enrolment until the day of delivery. Maternal blood hormone levels will be tested approaching delivery and the following day after birth. Behavioural information about breastfeeding will be collected 1 week afterwards.Ethics and disseminationThe study has been approved by the Ethics Committees of the Women’s Hospital School of Medicine at Zhejiang University and the Wuyi First People’s Hospital. Results will be disseminated through peer-reviewed journals or international academic conferences.Trial registration numberChiCTR2000038849.