This paper aims to develop a numerical model to predict heat stress of frefghter under low-level thermal radiation. The model integrated a modifed multi-layer clothing model with a human thermoregulation model. We took the coupled radiative and conductive heat transfer in the clothing, the size-dependent heat transfer in the air gaps, and the controlling active and controlled passive thermal regulation in human body into consideration. The predicted core temperature and mean skin temperature from the model showed a good agreement with the experimental results. Parametric study was conducted and the result demonstrated that the radiative intensity had a signifcant infuence on the physiological heat strain. The existence of air gap showed positive efect on the physiological heat strain when air gap size is small. However, when the size of air gap exceeds 6mm, a diferent trend was observed due to the occurrence of natural convection. Additionally, the time length for the existence of the physiological heat strain was greater than the existence of the skin burn under various heat exposures. The fndings obtained in this study provide a better understanding of the physiological strain of frefghter and shed light on textile material engineering for achieving higher protective performance.