Family members of schizophrenia patients play an important role in the daily life of schizophrenia patients, who bear the burden of long-term caregiving and are in a state of stress. The mechanism of social support in the primary caregiver group of schizophrenia on their subjective well-being was still unclear. To explore the mechanisms underlying the development of psychological well-being among primary caregivers with schizophrenia, this study conducted a short-term longitudinal design to examine the direct effect of social support on subjective well-being and the indirect effect of social support on subjective well-being through Just World Beliefs and Hope. 214 primary caregivers of patients with schizophrenia were enrolled. This study found that: (1) social support, just world beliefs, hope, and subjective well-being were significantly positively correlated; (2) social support significantly mediated subjective well-being through just world beliefs; (3) social support significantly mediated subjective well-being through hope;(4) Just world belief and hope play a chain mediating role between social support and subjective well-being. Conclusions: Just world beliefs and hope mediated the relationship between social support and subjective well-being in primary caregivers with schizophrenia. This study proved the psychological mechanism that just world beliefs and hope mediate the relationship between social support and subjective well-being, and provided theoretical support for improving the mental health of primary caregivers with schizophrenia.