Abstract: The reaction mechanisms on hydrogen generation are the base of studying the microcosmic measure to reduce or control hydrogen generation in the severe accidents for the pressurized water reactor plants. The quantum chemistry software Gaussian03 was used to study the reaction mechanism of iron with water at the B3LYP/6-311+G(d) theory model of quantum chemistry. The active energy was calculated. The results show that the reaction of iron with water is a total reaction, dividing into two elementary reactions. The reaction rate of the total reaction is determined by the second elementary reaction because of its higher active energy. It is concluded that the second elementary reaction should be the key elementary reaction to reduce the hydrogen generation.