Abstract: A kinetics model should be constructed to study the hydrogen behaviorin the severe accidents for the pressurized water reactor (PWR). The reaction mechanisms and the release rate of hydrogen are the bases of kinetics model. The quantum chemistry software Gaussian03 was used to study the reaction mechanism of zirconium-water based on the HF/3-21G theory model of quantum chemistry. The reaction rate constants were also calculated. The results show that the reaction of zirconium-water is an overall reaction, which is divided into four elementary reactions. The reaction rate of the overall reaction was determined by the second elementary reaction due to its slowest reaction rate constant. It is concluded that the second elementary reaction should be the key elementary reaction to reduce the hydrogen generation. The conservative results reveale that it is safety to study the hydrogen behavior in the severe accidents for PWR using the kinetics model constructed in this presented method.