Abstract: In order to improve the uillization efficiency of reject fly ash, the means of the scanning electron microscopy and the particle size analysis method were a pplied to study the activity of the reject fly ash. A variation relationship of the activity with the time was studied during the mechanical grinding process of the reject fly a sh. An activity stimulating effect of the fly ash was observed under the different mechanical grinding time and activated mechanism of the mechanical grinded reject fly ash was analyzed. The test results showed that when the grinding time reached at 20 min, the strength of the reject fly ash binder would be increased by 50% in max a nd when the grinding time continued and the strength would be obviously reduced. Therefore, the reject fly ash activity stimulated by the mechanical grinding should be controlled within 20 min and the continued grinding would not have a role to improve the activity of the fly ash. The reject fly ash activated mechanism by the mechanica | grinding would mainly include the physics effect, crystalline transition and chemical changes. Although the mechanical grinding could improve the activity of the fly as h, but could not stimulate the activity of the reject fly ash fundamentally.Therefore, to improve the binding of the fly ash, a comprehensive consideration should be requi red from many aspects and the most economic partical fineness and grinding time should be selected.