Prosocial risky behaviour (PRB) combines risk and prosociality, defined as risk-taking for the benefit of others or society. Its differences across subjective social classes and underlying mechanisms are unclear. Study 1 (n = 590) used a questionnaire to explore the relationship between subjective social class and PRB. Study 2 (n = 120) manipulated subjective social class and set risk levels in prosocial risky dilemmas to examine causal relationships and boundary conditions. Study 3 (n = 120) used cognitive computational modelling to clarify effects and explore cognitive mechanisms. Results showed the following: (1) subjective social class negatively predicts PRB, especially in the 30%–50% risk range; (2) low social class individuals have lower loss aversion, higher guilt aversion and value PRB more; (3) two decision preferences parallel mediate the relationship between subjective social class and PRB. These findings offer insights for mitigating social class differences’ negative impacts and enhancing prosocial levels in risky situations.