To reveal the influence of mechanical activation on the performance of fly ash,the microanalysis (the energy spectroscopy,XRD and SEM),the distribution size of particle of fly ash and cement paste intensity of various age for different grinding time were studied.The relationships of the activity and the composition of fly ash,microstructure and the distribution of particle size by mechanical activation of fly ash were obtained.The internal glass beads with activity were released by grinding fly ash for a certain time.The particle specific surface area was improved and the hydration reaction of the interface and the surface active center was increased by grinding.The granularity distributing of fly-ash trended towards optimization.The polar molecules or ions were easier to intrude into the internal cavity of the vitreous body.The active silica and alumina of fly ash were rapidly depolymerized.Each performance index of fly ash was increased before grinding for 20 min.Cement paste intensity of various age increased along with the grinding time,and the early strength increase range was big,but the later period intensity increase range hastened slightly.The internal part of vitreous of fly ash was destroyed if the fly ash continued to be ground and the activity of fly ash was reduced.It is suggested that Guozhuang\'s fly ash should be ground for 20 min.