Objective To explore the relationships between rumination, self-disclosure and posttraumatic growth in patients with breast cancer, and to provide reference for taking targeted measures to promote posttraumatic growth in patients. Methods The Event Related Rumination Inventory, Distress Disclosure Index and Posttraumatic Growth Inventory were used to investigate 200 patients with breast cancer. Results The total score of posttraumatic growth, rumination and self-disclosure were (61.17±12.69), (29.29±10.63), and (38.05±6.82), respectively. Invasive rumination was negatively correlated with posttraumatic growth and self-disclosure, while self-disclosure was positively correlated with posttraumatic growth (all P<0.05). Self-disclosure played a mediating role between invasive rumination and posttraumatic growth, and the mediating effect accounted for 40.18% of the total effect. Conclusion Self-disclosure is the mediating variable between invasive rumination and posttraumatic growth in breast cancer patients. Medical staff should guide patients to reduce negative contemplation, enhance their willingness to self-disclosure, and improve the level of post-traumatic growth.