This study examined the trajectory of probable PTSD prevalence and severity, and analyzed the predictors for PTSD severity in bereaved survivors at 6 months and 18 months after the 2008 Sichuan earthquake. This was a longitudinal study with 226 bereaved survivors sampled at 6 months and 18 months post-earthquake. The instrument used in the study was the revised version of the Impact of Event Scale. The results showed that the prevalence of probable PTSD in bereaved survivors decreased significantly from 38.9% at 6 months to 16.8% at 18 months post-earthquake. Loss of a child, being directly exposed to the death of family members and property loss during the earthquake, and mental health services utilization after the earthquake were significant predictors for PTSD severity at both assessments. These findings can contribute to post-disaster psychological rescue work. The bereaved survivors at high risk for more severe PTSD should be particularly targeted.