Hydrogen production via ammonia decomposition faces challenges due to low NH3 conversion, and the activity of ammonia decomposition catalysts requires improvement. Molecular sieves, with their large specific surface areas and well-developed pore structures, can serve as excellent carriers for ammonia decomposition catalysts to enhance their activity. However, the influence of molecular sieve carrier properties on ammonia decomposition remains unclear. Therefore, three types of molecular sieve carriers widely used in ammonia decomposition, namely ZSM-5, SBA-15, and MCM-41, were selected, and a series of Co-based molecular sieve catalysts were prepared by the equal-volume impregnation method. This work investigated the effect of Co-based catalysts with different molecular sieve carriers (ZSM-5, SBA-15, MCM-41) on hydrogen production from ammonia decomposition. The catalytic activity of the Co-based molecular sieve catalysts followed the order: Co/SBA-15>Co/MCM-41>Co/ZSM-5. Co/SBA-15 exhibited the best ammonia decomposition activity. The physicochemical properties of the Co-based catalysts were analyzed using characterization techniques including BET, XRD, SEM, H2-TPR, and NH3-TPD to reveal the changes in the molecular sieve carriers before and after loading and their effects on ammonia decomposition performance. The specific surface area, pore structure, surface morphology, catalyst particle size, and acidic sites were found to significantly influence the ammonia decomposition activity. In contrast, the redox capacity of the carriers and the metal grain size had a lesser impact. Based on the catalytic activity and characterization results, we concluded that an ideal carrier for an ammonia decomposition catalyst should possess a high specific surface area, a well-developed pore structure with an appropriate pore size, and a low density of weak acidic sites. The active sites of the ammonia decomposition reaction were associated with the metallic cobalt state, and H2 reduction treatment of the catalyst prior to the reaction could increase the number of active sites, thereby effectively improving its ammonia decomposition activity. Meanwhile, the Co/SBA-15 catalyst exhibited a lower apparent activation energy for ammonia decomposition compared to the other two catalysts. Additionally, the Co/SBA-15 catalyst exhibited excellent catalytic stability and reproducibility. Finally, in situ DRIFTS experiments on the Co/SBA-15 catalyst revealed the enrichment of —NH and —NH2 intermediates on its surface, indicating that the rate-limiting step in the ammonia decomposition reaction might be the recombinative desorption of adsorbed N atoms. This study elucidates how the physicochemical properties of catalyst carriers influence ammonia decomposition performance and provides theoretical guidance for selecting catalyst carriers for this reaction.