Background Wenchuan, China was damaged by a magnitude 8.0 earthquake in May 2008. After 15 years, there is still a lack of research addressing post-traumatic stress disorder (PTSD) among Wenchuan earthquake survivors. Few studies have examined the effects of online misinformation on the psychological mechanisms of PTSD in this population, especially during COVID-19, and little is known about their psychological state during such public health emergencies or how they deal with the associated internet misinformation deluge. Methods In this study, a mediated structural equation model was conducted among individuals who developed PTSD after surviving the Wenchuan earthquake (N = 356). Results The results show that PTSD had a positive effect on rumination and fear of missing out (FOMO) (beta = 0.390, p < .001; beta = 0.372, p < .001), and misinformation beliefs may be directly caused by rumination and FOMO (beta = 0.396, p < .001; beta = 0.420, p < .001). Additionally, this study examined the potential relationship between PTSD and misinformation beliefs via rumination (beta = 0.154, p < .001) and FOMO (beta = 0.156, p < .001). Conclusion This study makes a valuable contribution to the literature concerning the association between PTSD and misinformation beliefs and provides insight into how psychopathology-related variables are influenced by the use of social media platforms. This study also has significant implications for people surviving natural disasters from the perspective of intervention design and health enhancement. These findings serve as an important reference for the analysis of psychological variables in social media use among the PTSD population.