Study 1: 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? 45 participants will be allocated to MCC, WET, and WET+MBA on a 1:1:1 ratio. 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. Study 2: The study aims to examine the effectiveness of the online revised written exposure therapy (WET-R) for PTSD and subclinical PTSD among Chinese adults with a randomized controlled trial. The study will recruit 90 participants, with 60 randomized to the online WET-R group, and 30 randomized to the minimal contact control (MCC) group. The WET-R intervention consists of 5-7 one to one sessions. The primary outcome PTSD Symptom Scale - Interview for DSM-5 (PSSI-5) will be administered on baseline, post-treatment, 1-month follow-up, 3-month follow-up, and 6-month follow-up assessments.