Rice straw and Tiefa coal were used as experimental materials. Under the conditions of different temperatures and Cl/K molar ratio, the experiment was performed by using co-firing devices and ion measure instruments (flame photometer and a visible spectrophotometer). The results reveal that the majority of alkali metals and chlorine are released to gas phase by the precipitation of alkali chloride and HCl(g) above 700°C. Alkali metals release increase with the raising of chlorine content in raw materials. Alkali metals release related to sulfur, silicon and aluminum etc.