The very high-temperature gas-cooled reactor is a fourth-generation reactor with inherent safety and modular high-temperature characteristics. Utilizing ultra-high temperature heat to produce hydrogen by iodine-sulfur cycle is one of the important applications. In this process, the decomposition of hydroiodic acid determines the efficiency of hydrogen production, so it is important to design a hydroiodic acid decomposer with the compact structure and good heat preservation. In this paper, numerical simulations are performed on the hydroiodic acid decomposer used for the decomposition of hydroiodic acid. The species transport model, volume reaction and porous model are used to calculate the temperature field and the distribution of each component. The results show that the temperature of the catalytic zone meets the reaction requirements, the temperature drop in the reactor is about 55 K, and the overall decomposition fraction can reach 21.4%. The current flow rate has an acceptable performance for the improvement of the decomposition rate. The results of this study can provide theoretical calculation references for the engineering application of hydroiodic acid decomposers.