The elemental and phase compositions of cornstalk ash are largely different from that of lean coal ash. In comparison with pure lean coal, co-firing of cornstalk and lean coal may result in serious corrosion to the heating surface of superheater. This paper presents a composition comparison between the two ashes by means of Energy Dispersive Spectrometer (EDS) analysis and X-Ray Diffraction (XRD) analysis. A number of experiments were carried out in a tubular furnace to investigate the ash corrosion characteristics. Measured corrosion characteristic curves show the agreement of parabolic rule, and the corrosion is mainly effected by ash compositions and temperature. It was also found that alkali-chloride, alkali metal sulphide in ash and HCl in flue gas significantly aggravate the corrosion. The corrosion speed increases as the temperature increase, and becomes significantly high when the temperature is above 550°C.