Heat stress would be intensified under global warming and become a key issue of occupational health for labor force working outdoors. The changes in labor force would affect regional socioeconomic development. So far, changes in labor force due to heat stress are not well documented in China. In this study, heat stress based on wet-bulb globe temperature (WBGT), which combines the thermal effects on the human body of both temperature and humidity, is projected for the near future (2021–2050) and the end of the century (2071–2099). Changes in labor capacity are then estimated for heavy and light work based on the relationships between labor capacity and the WBGT. Low and high emission scenarios, namely Representative Concentration Pathway (RCP) 2.6 and RCP8.5, are considered for the future projections in the hottest two months (July and August) in China. Results suggest that the WBGT would increase by more than 3–5 °C by the end of the century. The labor capacity would decrease by more than 40% for both heavy and light work in considerable areas such as South and East China, where there is a large population and developed economy. This indicates that labor force would reduce significantly due to intensified heat stress. This study calls for special attention to the impact of heat stress on occupational health and the labor force in China in the future.