Objective: The risk for aid workers to develop posttraumatic mental problems highlights the importance of reducing the harm of posttraumatic stress symptoms (PTSS) and promoting the benefits of posttraumatic growth (PTG). This study examined the negative relationship between mindfulness and PTSS, and the positive relationship between mindfulness and PTG, and further explored the mediating roles of self-acceptance, intrusive rumination, and deliberate rumination. Methods: Aid workers from the Red Cross Organization (N = 298) were recruited in the study. They completed a series of questionnaires assessing mindfulness, self-acceptance, rumination, posttraumatic stress disorder (PTSD), and PTG. Results: Results showed that mindfulness was significantly and negatively associated with PTSD through positively predicting self-acceptance and negatively predicting intrusive rumination. Mindfulness was significantly and positively associated with PTG, although the mediating effect did not exist because the path between self-acceptance and deliberate rumination was impassable. Conclusion: The mechanisms between mindfulness, PTSD, and PTG were further explored. Limitations and implications for trauma intervention and future research were discussed.