This study analyzed the heat generation characteristics of a PEMFC stack for a compact firefighting special-purpose vehicle according to the temperature and relative humidity of the supplied air. The heat generation was predicted using AMESim at air inlet temperatures of 50°C and 80°C under various relative humidity conditions. The results showed a nonlinear trend in which heat generation initially decreased and then increased with increasing relative humidity at both temperatures. The minimum heat generation occurred at approximately 70% RH at 50°C and 50–60% RH at 80°C. In particular, heat generation increased noticeably above 60% RH at 80°C, indicating a significant effect of relative humidity under high-temperature and high-humidity conditions. These results can provide fundamental data for designing thermal management systems and determining the operating conditions of automotive PEMFC systems.