In order to rapidly achieve the goal of global net-zero carbon emissions, ammonia (NH3) has been deemed as a potential alternative fuel, and reforming partial ammonia to hydrogen using engine exhaust waste heat is a promising technology which can improve the combustion performance and reduce the emission of ammonia-fueled engines. However, so far, comprehensive research on the correlation between the reforming characteristic for accessible engineering applications of ammonia catalytic decomposition is not abundant. Moreover, relevant experimental studies are far from sufficient. In this paper, we conducted the experiments of catalytic decomposition of ammonia into hydrogen based on a fixed-bed reactor with Ru-Al2O3 catalysts to study the effects of reaction temperature, gas hour space velocity (GHSV) and reaction pressure on the decomposition characteristics. At the same time, energy flow analysis was carried out to explore the effects of various reaction conditions on system efficiency. The results show that both the ammonia catalytic conversion and decomposition efficiency increase with the reaction temperature increasing. However, these two parameters decrease with the increases of GHSV and reaction pressure, the former due to the reduction of ammonia retention time in the reactor as GHSV accelerates, and the latter due to the high-pressure environment inhibiting the overall reaction towards ammonia decomposition. In addition, the maximum conversion rate of 86% and a peak decomposition efficiency of 112% were achieved at 853 K, 2000 h-1, and 0.1 MPa. The energy flow analysis shows that increasing the reaction temperature increases the decomposition losses, but the total calorific value of the reformate increases, which is expected to improve the combustion efficiency of ammonia fueled engines and reduce the unburned ammonia emissions. Furthermore, GHSV has a negligible impact on decomposition losses. This paper contributes to the database of hydrogen production from the ammonia thermo-catalytic decomposition, analyze the energy flow distribution of the catalytic decomposition process, and provides important information for development of zero-carbon ammonia-hydrogen fueled engines.