Pyrolysis and combustion process of combustible materials is the initial stage of fire, which has greatly influence on the development of fire. Through changing atmosphere in the halfway, the pyrolysis and combustion properties of plywood were studied by using a thermogravimetric analyzer in nitrogen under constant temperature conditions. The result indicates that effect of transforming atmosphere on the pyrolysis is little before the end of the first stage of plywood pyrolysis. In the second phase of plywood pyrolysis, atmosphere changing from air to nitrogen, the trend of weight loss is along with the trend of nitrogen atmosphere. After changing from nitrogen atmosphere to air atmosphere, pyrolysis rapidly accelerates, and final rate of weight loss is the same as that in air atmosphere.