Hydrogen-rich gas production from pyrolysis of biomass in an autogenerated steam atmosphere was proposed. The scheme aims to utilize steam autogenerated from biomass moisture as a reactant to react with the intermediate products of pyrolysis to produce additional hydrogen. In the present Article, the effect of reactor temperature, moisture content, heating rate, and sweeping gas flow rate on the process was investigated experimentally. Measurement of the atomic structures of chars produced from pyrolysis of wet/predried biomass was carried out with XRD. The results show that heating rate is a key role in the process. Under fast-heating conditions, drying and pyrolysis occurred in a relatively shorter time, which enhances the interactions between the autogenerated steam and the intermediate products and hence produces more hydrogen. Under slow-heating conditions, however, the autogenerated steam from moisture will be partially purged away from the reaction zone, leading to a weakened effect on the subsequent reactions. The use of sweeping gas is unfavorable to hydrogen production due to the reduced residence time of both the autogenerated steam and the volatile. Moisture content has a great effect on hydrogen production. The H-2 yield and content increases with the moisture content. Under the conditions of fast-heating rate and without the use of sweeping gas, the pyrolysis of B-W (wet biomass with a moisture content of 47.4%, wet basis) exhibits higher H-2 yield of 495 mL/g, H-2 content of 38.1 vol %, and carbon conversion efficiency of 87.3% than those (267 mL/g, 26.9 vol %, and 68.2%) from the pyrolysis of B-TD (the predried biomass with a moisture content of 7.9%, wet basis). The comparison of atomic structure of chars from B-W and B-TD further confirms that directly pyrolyzing wet biomass without predried treatment favors the pyrolysis to a deeper extent.