The influence of specific surface area on cementitious reactivity of phosphorous slag power(PS)was studied and the grey relation degree and relation polarity between the particle size distribution characteristics and the activity coefficients of PS were investigated. The results indicated that particles could weaken the hydration activity of PS and when particle size is greater than 30.2 μm and enhance that when smaller than 30.2 μm. The volume fraction of particles with size of 5.0~10.0 μm and 10.0~20.0 μm are important determinant of the early activity index(i.e. A7)and activity index A28，respectively. Therefore，specific surface area should be greater than 397 m 2·kg -1 to improve the hydration activity of PS and it is important to increase the content of particles with size of 5.0~30.2 μm，especially with size of 5.0~20.0 μm and decreasethat of particles bigger than 30.2 μm.