Paper studies on phosphorus slag powder(PSP) processed from phosphorus slag with different open-air storage time, analyses the changes in the performance of PSP and the impact on properties of cementitious materials. The results showed that with the growth of open storage time, vitreous structure of PSP would gradually be decomposed, part of the vitreous would transform into crystal form. Density of powder would diminish and ignition loss increase. Activity of PSP would slowly decreases, and the activity differences main reflected after 90d. Concerning on the point of concrete strength rating, 10 years phosphorus slag can still be used. The above provide a basis for resource utilization of phosphorus slag with long storage time.