The sulfuric acid decomposition reactor is an important equipment in the thermochemical sulfur iodine hydrogen production system, its heat transfer needs to match the hydrogen production capacity of the system. In order to study the effect of different structures ofsulfuric acid decomposition reactors on heat transfer, and to ensure that the heat transfer of the reactor meets the system requirements whilealso meeting the limitations of manufacturing processes. Through experiment, the reaction kinetics parameters of sulfuric acid decomposition reaction were calibrated and a reaction kinetics model was established. The reactor was simulated by gPROMS to obtain parameterssuch as pressure, temperature, flow rate, and component concentrations within the reactor. Results show that the total conversion rate cannot be improved by adjusting the length ratio of the preheating section and the reaction section or increasing the thermal conductivity of thepacked particles, while the total length of the reactor remains unchanged. Increasing the length of the preheating section in the reactor cansignificantly increase the total conversion rate. The key reason is that the length of the preheating section determines whether the temperature inside the reactor can reach the optimal temperature of 850 ℃ required for the SO3 decomposition reaction. Reducing the reactor diameter does not increase the total conversion rate, although reducing the diameter of the reactor is beneficial for heat transfer, due to the unchanged inlet flow rate, the fluid flow rate increases significantly, reducing the residence time of the reactants and significantly increasingthe reactor flow resistance. Using a sleeve annulus internal and external heating structure as the preheating section of the reactor can effectively improve the total conversion rate. When both internal and external heating are used, increasing the heat transfer area is beneficial forshortening the length of the preheating section. The length of the preheating section requires about 900 mm to achieve a reactor outlet temperature of 850 ℃ . A reactor structure design that meets the requirements has been found.