Abstract: Objective To explore the changing tendency of post-traumatic stress disorder(PTSD) and perceived social support(PSS) and the relationship betw een them among secondary school students in Wenchuan after the May 12 earthquake.Methods A total of 1 966 students from three secondary schools of Wenchuan were evaluated with PTSD ChecklistCivilian Version(PCL-C) and Perceived Social Support Scale(PSSS) three times after the earthquake.Results The general incidence rate(χ²=66.622,P< 0.001) and severity (χ²=70.762,P< 0.001; Gamma=-0.168,P< 0.001) of PTSD of the three surveys show ed a decreasing tendency.The overall average score of PSSS for the three surveys show ed no difference with a mean of 59.19±11.08.Logistic regression analyses show ed that the comprehension of all aspects of social support was a protective factor for PTSD.Conclusion Stable social support can promote mental health of PTSD patients and should be strengthened especially among the students with positive PTSD symptoms.