Thermal cooling mechanisms of human exposed to electromagnetic (EM) radiation are studied in detail. The EM and thermal co-simulation method is utilized to calculate the and temperature distributions. Moreover, Pennes' bioheat equation is solved to understand different thermal cooling mechanisms, including blood flow, convective cooling, and radiative cooling separately or jointly. The numerical results demonstrate the characteristics and functions for each cooling mechanism. Different from the traditional view that the cooling effect of blood is usually reflected by its influence on sweat secretion and evaporation, this paper indicates that the blood flow itself is an important factor of thermal cooling, especially for high-intensity EM radiation. This paper contributes to the fundamental understanding of thermal cooling mechanisms of human.