Posttraumatic stress disorder (PTSD) is a significant condition among breast cancer survivors (BCSs). However, few intervention studies for cancer-related PTSD were conducted among Asian cancer survivors. We evaluated a culturally sensitive expressive writing intervention, which combined cognitive reappraisal and emotional disclosure, in reducing PTSD among Chinese American BCSs. We also tested social constraints (defined as social conditions when individuals feel misunderstood or alienated when they desire to disclose their thoughts and feelings) as a moderator.Chinese American BCSs (n = 136) were randomly assigned to three groups with assigned writing topics for 3 weeks: a self-regulation group, which wrote about the deepest feelings related to cancer in week 1, cognitive reappraisal about stress and coping in week 2, and benefit finding in week 3; an enhanced self-regulation group, with the same instructions, except weeks 1 and 2 were reversed; and a cancer-fact group, which wrote about cancer experiences objectively for 3 weeks. PTSD symptoms were measured at baseline and 1-, 3-, and 6-month follow-ups. Social constraints were measured at baseline.Both the self-regulation and enhanced self-regulation groups showed reduced PTSD symptoms compared to the cancer-fact group. For reexperiencing and hyperarousal symptoms, expressive writing was more effective for BCSs who experienced high vs. low levels of social constraints; the opposite was found for avoidance symptoms.Findings demonstrated the effectiveness of expressive writing intervention in reducing PTSD for this minority population, and that the moderating role of survivors' social network varies among different PTSD symptom clusters. ClinicalTrials.gov Identifier: NCT02946619.