Abstract:: Objective To observe the effect of online mindfulness-based stress reduction (MBSR) training on mental health of postoperative patients with breast cancer. Methods Postoperative patients with breast cancer were randomly divided into experimental and control groups, with 95 cases in each group. Both groups received conventional nursing, and 8 weeks of online MBSR training were added in experimental group. The scores of positive/negative psychology and post-traumatic growth were compared between two groups before/after intervention and at 3-month follow-up. Results In experimental group, scores of positive psychological resources (resilience, self-efficacy), post-traumatic growth, appreciation of life, new possibilities, personal strength, spiritual change, and interpersonal relationship were higher, while scores of negative psychology (fear of disease progression, depression) were lower after intervention (P <0.05). Compared with control group, scores of self-efficacy, resilience and post-traumatic growth were increased, and scores of nosophobia and depression decreased in experimental group (P <0.001). Conclusion Online MBSR training can effectively improve the positive psychological level, alleviate negative psychological level, and promote post-traumatic growth level in postoperative patients with breast cancer.