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The influences of water media on the hydrogen isotopic composition of organic-thermogenic natural gas were tested in three series of experiments on coal pyrolysis, with no water, deionized water ( $\delta D_{H_2 O} $=−58‰), and seawater ( $\delta D_{H_2 O} $=−4.8‰) added, respectively. The experimental results show that the productivities of H and CO obviously increased under hydrous conditions and that the productivity of CH also remarkably increased in the high-evolution phase of hydrous experiments. Water was involved in the chemical reaction of hydrocarbon generation, and then the hydrogen isotopic composition of methane was affected. There is a linear correlation between the hydrogen isotopic composition of methane and its productivity, as reflected in the three series of experiments. In the case of the same CH productivity, the hydrogen isotopic composition of the methane produced in anhydrous experiments was the heaviest, that of the methane produced in seawater-adding experiments came second, and that of the methane produced in deionized water-adding experiments was the lightest. The hydrogen isotopic composition of natural gas/methane is affected by the following factors: 1) the characteristics of hydrogen isotopic composition of organic matter in source rocks, 2) the thermal evolution extent of organic matter, and 3) fossil-water media in the natural gas-generation period. The experimental results show that the influence of the fossil-water medium in the natural gas-generation period was lower than that of the other factors. [ABSTRACT FROM AUTHOR]