Abstract: Air-cooled microreactor is a highly integrated system with the advantages of easy transportation and reliable operation, which can meet the demand for energy in remote areas. The heat removal capacity of the non-energetic air-cooled waste heat removal system is an important indicator for evaluating the performance and safety of air-cooled microreactors. It uses air as the final heat trap and has the advantages of simple system equipment and reliable long-term operation. Some nuclear power units utilize the air-cooled waste heat removal system to export heat and control the core temperature under the conditions of coolant loss accident and decay heat removal after reactor shutdown. The purpose of this paper is to study the coupled heat transfer characteristics of the natural cycle of the air-cooled waste heat removal system and the influence of different insulation layer structural characteristics on the flow heat transfer capacity of the waste removal system, and to propose an air-cooled waste heat removal system using a horizontal pressure vessel with a double-layer thermal insulation structure, analyze the heat-carrying process of the waste heat removal system, put forward the corresponding assumptions in order to simplify the solving region and the physical model, and determine the masking effect of the radiant heat transfer based on the vectorial method, supplemented by reasonable constitutive equations and mathematical models. The coupling between radiative heat transfer and thermal conductivity, convective heat transfer and heat transfer and flow of the waste heat removal system was realized by supplementing the reasonable constitutive equations and mathematical model to complete the closure of the control equations, and a one-dimensional analysis program was developed to simulate the flow and heat transfer characteristics of the waste heat removal system, and the accuracy of the model was verified by using the experimental data, and based on the program, the outer flow channel closure (thermal insulation) and opening (insulation boundary condition) and the outer flow channel closure (insulation boundary condition) and opening (insulation boundary condition) of the waste heat removal system were discussed in the boundary condition of the fixed-wall temperature of the pressure vessel. Based on this program, the effects of closed (insulation boundary conditions) and open (insulation non-insulation boundary conditions) outer flow channel on the accuracy of the model calculation were discussed, and the relationship between the natural circulation characteristics and the change of the outer wall temperature of the pressure vessel, the emissivity, the width of the annular cavity, the ambient temperature, and the equivalent thermal resistance of the insulation layer were calculated. The results of the study show that appropriately increasing the outer wall temperature of the pressure vessel, the emissivity of the insulation layer and the width of the annular cavity, and appropriately decreasing the equivalent thermal resistance of the insulation layer can help to improve the system’s waste heat removal capacity, and the system’s waste heat removal capacity decreases with the increase in ambient temperature.