Background Posttraumatic stress disorder (PTSD) and subthreshold PTSD are still major global concerns, especially in developing areas short of mental health resources. Written exposure therapy (WET), a brief 5-session treatment, has been found to be effective in reducing PTSD symptoms, but no studies have examined it in an Eastern context. Mindfulness-based meditation mobile application may be a promising approach to reduce insomnia comorbid with PTSD. The current study aims to: 1) examine the effectiveness of WET for Chinese PTSD and subthreshold PTSD patients, and 2) examine the effectiveness of adding a mindfulness-based application (MBA) to WET for reducing comorbid insomnia. Methods The randomized controlled trial will enroll 150 adults with subthreshold/full PTSD and comorbid insomnia. Participants will be randomly assigned to written exposure therapy plus mindfulness-based application condition (WET + MBA, n = 50), written exposure therapy alone (WET, n = 50), or minimal contact control (MMC, n = 50). Clinical interview of the primary outcome (PTSD symptoms) will be administrated at baseline, posttreatment, 3- and 6- month follow-up, while self-reported PTSD symptoms and secondary outcomes (insomnia severity) will be administrated at baseline, every week and all follow-ups. Discussion This is the first study applying WET in Chinese PTSD patients, as well as examining a mindfulness-based mobile application as a treatment add-on for comorbid insomnia. Study findings will contribute to the knowledge of the effectiveness of WET and a mindfulness-based mobile application, and the development of a culture-adapted treatment protocol. Trial registration ChiCTR, ChiCTR2000034119. Registered 24 June 2020, http://www.chictr.org.cn/showproj.aspx?proj=55,467 .