Background: The outbreak of the COVID-19 pandemic has caused extensive public health concerns, posing significant challenges to healthcare services. One particular area of concern is the mental health of psychiatric patients, who are often a neglected group. The aim of this study was to investigate the prevalence of, and associated factors for symptoms of post-traumatic stress disorder (PTSD) among psychiatric patients in China during the COVID-19 pandemic. Methods: Self-reported questionnaires were distributed to patients in four psychiatric hospitals in Beijing, China, between April 28th and May 30th, 2020. Information regarding sociodemographic characteristics, COVID-19 related factors, support, psychosomatic factors, and PTSD symptoms was collected data using a series of scales, such as the Impact of Event Scale-Revised, the 7-item Generalized Anxiety Disorder Scale, the 9-item Patient Health Questionnaire depression scale, and so on. Multivariate regression was used to identify factors related to PTSD symptoms. Results: A total of 1,055 psychiatric patients were included in the final sample. The prevalence of PTSD symptoms was 41.3%. Hierarchical linear regression demonstrated that fear of the pandemic and anxiety were shared associated factors for both symptoms of PTSD and its subscales. Additionally, age was an associated factor for the total PTSD (β = 0.12, p < 0.01), intrusion (β = 0.18, p < 0.001), and avoidance (β = 0.1, p < 0.05) symptoms; depression was an associated factor for the total PTSD s (β = 0.13, p < 0.001), intrusion (β = 0.11, p < 0.01), and hyperarousal (β = 0.19, p < 0.001) symptoms. Conclusions: The prevalence of PTSD symptoms was high among psychiatric patients during the COVID-19 pandemic in China. This study found that age, fear of the pandemic, anxiety and depression are significant associated factors of PTSD symptoms in psychiatric patients during the pandemic. We call for higher awareness and introduction of PTSD interventions to relieve the psychological stress in these patients.