The goal of this study is to use a pilot RCT to assess the effectiveness of Written Exposure Therapy (WET) and a mindfulness-based app (MBA) for managing PTSD and comorbid insomnia in China. The main questions it aims to answer are: 1. Does WET alone reduce PTSD symptoms in Chinese patients with PTSD and subthreshold PTSD? 2. Does the addition of MBA to WET lead to greater reductions in comorbid insomnia symptoms compared to WET alone? Researchers will compare WET alone and MCC to see if WET provides benefits in managing PTSD. Researchers will compare WET plus MBA and WET group to see if the integrated MBA treatment provides additional benefits in managing insomnia. Participants will: * Undergo random assignment to one of three groups: WET, WET plus MBA, or MCC. * Receive clinical interviews for primary outcomes (PTSD symptoms) at baseline, posttreatment, and during follow-ups. * Provide self-reported data on PTSD symptoms and insomnia severity at multiple time points. * In the WET plus MBA group, additionally use a mindfulness-based app as part of their treatment.