The influence of water on gas generation from humic type organic matter at highly to over mature stage was investigated with thermal simulation experiments at high temperature and pressure. The result of the experiments indicates that the effect of water on gas generation was controlled by the thermal maturity of organic matter. Water could enhance gas generation and increase hydrocarbon gas yields significantly at over mature stage of humic type organic matter. Hydrogen isotopic compositions of coal-derived gases generated at highly to over mature stage were mainly controlled by thermal maturity of source rocks, but also affected by formation water. Highly and over mature coal measure source rocks are widely distributed in China. The hydrocarbon gas generation capacity of coal measure source rocks and resource potential of coal-derived gases in deep formations would be significantly enhanced assuming that formation water could be involved in the thermal cracking of highly to over mature organic matter in real geological settings.