Under the schedule of steaming and high temperature drying designed, by using traditional chemical analysis and GC, the author has studied the chemical constitutions of Larch (Larix gmelini)samples before and after drying. The results showed that the steaming and high temperature drying to the wood is a combination process of chemical and physical changes The wood undergoes not only such physical changes as diffution of watery, but also a series of chemical changes The acetyl groups in hemicellulose fall off to formulate acetic acid, increasing the acidity of wood, and thus putting the wood under mild weak acid environment The chemical linkages between lignin and hemicelluloses degraded hydrolically, resulting from the acid effect.