A downward-illuminated 100 W LED was studied herein, and its temperature and thermal stress distributions were calculated under three conditions: pure thermal conduction, thermal radiation and thermal conduction coupling, and convection, thermal radiation, and thermal conduction coupling. The following factors were evaluated: temperatures of the characteristic points on the lens’ external surface; the effects of air convection in the closed cavity of lens and surface radiation on the lens’ temperature distribution; the effect of surface emissivity on the central temperature of the lens; the effect of the thermal expansion coefficient on the maximum thermal stress. Results show that thermal convection produces a negligible rise in temperature (less than 1%), whereas surface radiation results in a 13.3% temperature rise at the lens’ center. The temperature of the lens’ center varies approximately linearly with the light source’s emissivity and the lens surface area, whereas the maximum thermal stress varies linearly with the thermal expansion coefficient. The maximum thermal stress is concentrated at the lens’ corners, whereas the maximum total deformation displacement is concentrated at its center. Therefore, when designing high-power LED lenses, a low-emissivity coating should be considered to reduce the lens’ temperature while still satisfying the optical requirements. To reduce thermal stress and deformation, a material with a small thermal expansion coefficient should be used and the lens’ corners should be avoided.