Reducing carbon emissions in sintering process is a crucial component for achieving green, low-carbon, and high-quality development in long process of iron and steel industry. The concept of carbon-free sintering provides a new approach for reducing carbon emissions in sintering, and utilizing hydrogen-based fuels for gas-based carbon-free sintering can achieve zero carbon emissions; however, the combustion of hydrogen fuel introduces a water vapor atmosphere, which affects sinter formation. Through micro-sintering experiments, the effect of oxygen volume fraction and water vapor volume fraction on sinter formation was studied in this paper. The results show that in O₂-N₂atmosphere, with the oxygen volume fraction increases, the compressive strength of the sample rises from 965 N to 1 360 N, the proportion of hematite increases, while the proportions of magnetite and porosity decrease, and the FeO content decreases from 14.47% to 2.6%. In H₂O(g)-O₂-80%N₂atmosphere, with the water vapor volume fraction increases from 0 to 20%, the compressive strength of the sample decreases from 1 360 N to 825 N, the proportions of hematite and calcium ferrite decrease, while the proportions of magnetite and porosity increase, and the FeO content increases from 2.6% to 13.07%. In H₂O(g)-15%O₂-N₂ atmosphere, with the water vapor volume fraction increases, the compressive strength of the sample decreases from 1 224 N to 962 N, and the FeO content decreases from 3.11% to 2.82%. In the mixed atmosphere of oxygen and water vapor, the oxidation effect is stronger, and when the water vapor mass fraction is less than 20%, its effect on the sintered sample is not large, so it is feasible to use hydrogen-based fuels for carbon-free sintering, which has greater potential for carbon emission reduction, but it is required to reasonably control the air-fuel ratio.