Abstract: Fly ash can be used to improve the performance of cement,especially to effectively reduce the total heat of hydration in cement.Although the post-strength of cement is high,it has low hydration activity in early stage.When the temperature is low,the cement has no activity at all.If more fly ash is added,the post-strength of the cement will decrease dramatically and the fly ahs will be strictly restricted in oil well cementing.Since there is no hydration reaction between fly ash and no strength is generated,so activator needs to be used to activate its activity.The key to research fly ash is to study its reactive activator.So a lot of lab test are conducted.Results show that J-1,a reactive activator in combination of inorganic salts,can effectively activate the activity of fly ash.It enables the ratio of fly ash to cement mixed to reach more than 1:1,and it may be close to 2:1 if being applied to surface cementing.The early strength of fly ash can be kept high.Meanwhile,the cement will be expanded to enhance the interface cementing strength.What’s more,the particles of fly ash are bigger than those of cement,which is favorable for sealing.