Background: As a result of the COVID-19 pandemic and its far-reaching impact, the prevalence of posttraumatic stress disorder (PTSD) symptoms is increasing significantly in China. Yet access to reliable and effective psy-chological treatment is still limited during the pandemic. The widespread adoption of mobile technologies may provide a new way to address this gap. In this research we will develop an Acceptance and Commitment Therapy (ACT) based intervention delivered by mobile application and will test its usability, efficacy, and mechanism of its effects in relieving PTSD symptoms.Methods: A total of 147 Chinese participants with a diagnosis of PTSD according to the Clinician Administered PTSD Scale (CAPS-5) will be randomly assigned to an intervention group (app-delivered ACT), an active com-parison group (app-delivered mindfulness), or a waitlist group. Participants in the intervention group or com-parison group will use their respective apps for one month. Online self-report questionnaires will be used to assess the primary outcome of PTSD symptoms and the secondary outcomes symptoms of depression, symptoms of anxiety, and posttraumatic growth. The potential mediating variable to be tested is psychological flexibility and its components. These assessments will be conducted at baseline, at five times during treatment, at the end of treatment, and at 1-and 3-month follow-ups.Discussion: As far as we know, this study is the first randomized controlled trial to investigate the usability, ef-ficacy, and mechanism of an app-delivered ACT intervention for PTSD. Furthermore, the research will assess the effect of treatment in reducing dropout rates, explore effective therapeutic components, and investigate mech-anisms of symptom change, which will be valuable in improving the efficacy and usability of PTSD interventions. Trial registration: ChiCTR2200058408.