The sulfuric acid decomposer is the key equipment of the iodine-sulfur cycle to achieve hydrogen production utilizing high temperature of very high-temperature gas-cooled reactor (VHTR). This study discusses sulfuric acid decomposer design and pipeline layout to improve the performance of the sulfuric acid decomposer based on a shell-and-tube heat exchanger with a bayonet heat exchanger as the core. The finite volume method was used to numerically calculate the heat transfer, temperature distribution, baffle layout, and sulfur trioxide decomposition rate in the sulfuric acid decomposer. The results show that the convective heat transfer coefficient of the fully developed section of the sulfuric acid decomposer for the given conditions is about 10 W center dot m(-2)center dot K-1 and the average temperature of the sulfuric acid in the catalyst region is about 1040 K. For the design, the inlet manifold plate effectively distributes the inlet helium, while the staggered baffles in the shell and tube heat exchanger increase the heat transfer area and improve the helium distribution to enhance the heat transfer. The prediction shows that the sulfur trioxide decomposition rate is about 60% in the catalyst region under design VHTR condition, which is in agreement with experimental measurement under corresponding temperature and space velocity. The results provide a reference for the design of sulfuric acid decomposers for VHTR.