Thermal analysis (included TG, DTG, and DSC) of untreated and treated wood was carried out to study the influence of fire retardants on the pyrolysis The results of TG and DTG showed that the temperature range and the rate of weight losses of wood in the stage of severe thermal degradation were changed, and the weight of remains at temperature of 400℃ was obviously increased when fire retardants were added. In temperature range of 20-450℃, in the DSC curve of untreated wood appeared three peaks cortesponding to the three stages of pyrolysis. The position and shape of DSC curws were essentially changed, by treating with fire retardants, that is, the combustion properties of wood were changed. Based on results, a primary explanation is offered on the mechanism of fire retardancy.