Rape straw was dried naturally after washing and respectively calcined 1, 3, 5, 7 and 9 hours at 400, 500, 600, 700, 800 and 900℃. The color and the appearance of the ashes were observed, and phase analysis and ingredients detection were conducted using XRD and EDS-SEM. Results showed that compared with the non-washing, washing can reduce the alkali metal and volatile substances of the straw by about 60%, which effectively improved the ash melting point, reduced slagging rate and increased the relative content of the SiO2. Thus, the optimal experimental condition to activate potential hydraulicity of SiO2 in ash of rape straw was calcining 5 hours at 500 ℃ using washed rape straw.