Social isolation has been shown to contribute to negative health outcomes. Within a Chinese cultural context where people highly value social relationships, family caregivers of people with severe mental illness are prone to experience social isolation due to demanding caregiving tasks and prevalent social stigma. However, an instrument that could comprehensively measure social isolation in Chinese family caregivers is lacking. The six-item Friendship Scale was designed to measure social isolation but it has not previously been validated for use in the Chinese context. This study aimed to assess the reliability and validity of the Friendship Scale - Chinese version (FS-C) in family caregivers of people with severe mental illness (SMI) (N = 200). Confirmatory factor analysis affirmed FS-C's single-factor structure. FS-C demonstrated good internal consistency (Cronbach's alpha = 0.74) and test-retest reliability (ICC = 0.72). Convergent validity was supported by moderate to strong correlations with social networks (r = 0.342, p < 0.01) and loneliness (r = -0.608, p < 0.01). Divergent validity was confirmed by a weak correlation with perceived social stigma (r = -0.190, p < 0.01). FS-C is a reliable and valid tool for assessing social isolation among Chinese family caregivers of people with SMI.