Post-traumatic growth (PTG) refers to the positive psychological changes experienced by individuals after struggling with major life crises or extremely challenging traumatic events, which exists widely in people exposed to natural disasters. The development of PTG is affected by many personal and social factors, particularly social support. Studies showed that social support can help individuals reduce their negative assessment of negative events and establish positive cognitive schemata. For individuals, perceiving more social support was conductive to the formation of PTG. However, the mechanisms in the relationship between perceived social support and PTG was not clear and needed more research. According to relevant theories, gratitude and self-efficacy play an important role. First, gratitude is a positive psychological quality related to individual cognition and emotion, which contains the meaning of grace and pleasure. According to the theory of gratitude expansion construction, gratitude can broaden the individual’s cognitive level and scope of thinking and action, help the individual build persistent resources such as body, intelligence, psychology and society, and effectively alleviate the individual’s adverse physiological reactions caused by negative cognitive emotions, so as to promote self-development. Second, self-efficacy refers to the individual’s speculation, judgment, and belief on whether he can complete a certain task. Self-regulation shift theory suggests that if post-traumatic individuals can get good social support, they will feel the improvement of self decision-making belief, improve their perception and evaluation ability, and promote positive psychological adaptation and growth. Therefore, this study incorporates gratitude and self-efficacy into the exploration of the relationships between social support and PTG, examining the internal mechanism under which perceived social support may affect PTG. This study used the Perceived Social Support Scale, the Gratitude Scale, the Self-Efficacy Scale and the Post-Traumatic Growth Inventory to investigate 476 youth participants who survived 12 years after the Wenchuan earthquake, 55.46% males and 44.54% females (M age = 24.69 years, SD = 4.25). The structural equation model was performed for analysis. The results showed that: (1) In the long period after the Wenchuan earthquake, the PTG of young survivors still existed, and in the relatively upper-middle level (M = 89.51, SD = 23.00); (2) Perceived social support significantly and positively predicted PTG (β = 0.43, p < 0.001); (3) Gratitude (95% CI [0.00, 0.08]) and self-efficacy (95% CI [0.04, 0.15]) played a partially mediating role between perceived social support and PTG, respectively. The current study further explored the mechanism between perceived social support and PTG, and revealed the important role of gratitude and self-efficacy in the relationship between them. In term of applications, this study found that social support is not only very important for promoting PTG in a short period after the earthquake, but also very important for the long-term development of PTG, which provides an important empirical basis for long-term post disaster psychological reconstruction and psychological intervention. Moreover, this study also provides practical enlightenment for the path of post disaster psychological intervention, that is, the psychological social intervention technology of providing social support, cultivating gratitude and improving self-efficacy, which has strong practical significance for improving the psychosocial adaptability of people in post-earthquake disaster areas.