[Objective] Firefighting gear serves as a crucial barrier for firefighters to prevent skin burns and reduce life-threatening risks during fire emergencies. Scientifically equipping firefighters with appropriate gear and enhancing the capability to assess on-site risk of disaster incidents are crucial for reducing casualties among firefighters. During a fire, the core of the heat transfer process revolves around the system of environment, clothing, and human body. A comprehensive analysis of the heat transfer mechanisms of the system is a scientific approach to improve the thermal protection level of the human body. This study aims to reveal the heat transfer mechanisms of the clothed human body to enhance occupational protection for firefighters.[Methods] The flame manikin system was employed in physical experiments to establish and validate numerical models. A geometric model of the combustion chamber with a clothed human body was established using a three-dimensional body scanner and reverse engineering technology. The turbulence, combustion, and radiation models were determined based on combustion chamber characteristics. The initial conditions, boundary conditions, and solution methods for the model were determined to calculate the system's heat transfer process. A skin heat transfer model with actual skin thickness was developed to predict skin burns. This model was combined with the flame manikin model to conduct virtual flame manikin tests under various thermal conditions. The fire environment and clothing are crucial elements in human thermal protection research. Herein, heat flux intensity and heat exposure duration were selected as environmental factors. Clothing thickness and surface emissivity were selected as clothing factors. All factors were set at nine levels, and a four-factor, nine-level orthogonal experimental design was used with 81 simulation calculations.[Results] The results of the simulation research indicate the followings. (1) The increase in heat flux intensity has the most substantial impact on the proportion of skin burns. (2) The proportion of skin burns generally increases with the increase in heat exposure duration. When the heat exposure duration is long, burn injuries tend to concentrate on the upper body because of the accumulation and increase in hot airflow. (3) With the increase in clothing thickness, the variation in the proportion of first-degree burns on the body's skin surface exhibits fluctuations. The proportion of second-degree burns considerably decreases between clothing thickness levels 1 and 3. (4) No clear pattern was observed in the proportions of skin burns with increasing clothing surface emissivity.[Conclusions] Determining a safe working distance or safe working time is important for the occupational safety of firefighters. The impact of clothing factors on burn proportions differs from the results of fabric studies. In the clothed state, different angles between the local body surface and heat source affect the actual incident heat flux on the clothing surface. Systematic analysis of burn degree distribution reveals that body areas with clothing openings, such as the neck, forearm, and calf, are the most susceptible to burns. Enhancing the design of clothing openings and improving the thermal protective performance of firefighter gear can benefit firefighters.