Background <p>This study aimed to test whether surrogate decision-makers’ (SDMs’) life orientation predicts their advance care planning (ACP) engagement, and whether this effect is mediated by perceived social support and uncertainty about the patient’s illness.</p> Methods <p>We conducted a cross-sectional survey of 276 adult SDMs of advanced cancer patients at a Chinese medical center. Participants completed validated Chinese scales for life orientation, social support, uncertainty of disease, and ACP engagement. A serial mediation model tested whether SDMs’ life orientation influenced ACP engagement indirectly through sequential effects on social support and uncertainty of disease, while controlling for relevant covariates. Descriptive statistical analysis, correlation analysis, and structural equation model analysis were conducted using SPSS 26.0.</p> Results <p>The results showed that more optimistic SDMs reported stronger social support and lower uncertainty of disease. Mediation analysis confirmed that higher life orientation was associated with greater ACP engagement both directly and indirectly: optimism was associated with higher perceived social support and lower uncertainty of disease, which in turn were associated with greater ACP engagement. The full model accounted for 57.8% of the variance in ACP engagement.</p> Conclusion <p>Consistent with our cross-sectional design, SDMs’ optimism was associated with more active ACP engagement, partly via higher perceived social support and lower illness uncertainty. Interventions that cultivate a positive outlook, enhance perceived social support, and clarify illness-related information may increase ACP uptake. The results underscore the importance of translating positive attitudes into concrete planning behaviors.</p>