Ammonia　is　a　carbon-free　energy　carrier　with　17.6　wt%　hydrogen　content.　The　design　of　an　efficient　and　compact　ammonia　decomposition　reactor　based　on　low-temperature　catalysts　is　the　key　to　realizing　industrial　hydrogen　production　from　ammonia.　In　this　work,　a　multiscale　model　was　developed　by　bridging　the　particle-scale　characteristics　of　catalysts　and　reactor　performances,　to　fully　comprehend　the　ammonia　decomposition　process.　The　effects　of　catalyst　porosity　and　pore　diameters　on　the　reactor　size,　precious　metal　loading,　and　the　profile　of　temperature　and　heat　flux　were　systematically　evaluated.　An　improved　reactor　design　was　further　proposed　by　applying　the　segmented　reactor　packed　with　two-stage　egg-shell-type　low-temperature　catalysts,　which　decreased　the　precious　metal　usage　by　61.6%　and　the　temperature　drop　by　42.9　K.　This　segmentation　strategy　balanced　the　reaction　rate　and　heat　flux,　indicating　a　significant　potential　in　highly　efficient,　economical,　and　reliable　hydrogen　production　from　ammonia.