Phosphorous slag, one of the pozzolanic active materials, is a by-product from the production of yellow phosphorous. In this paper, phosphorous slag was ground and adopted to prepare concrete as a mineral admixture. Specific strength method was used to assess the influences of phosphorous slag content on the pozzolannic effects in concrete with different age. The results indicated that with the increase of phosphorous slag content and age, the pozzolanic effects were enhanced in concrete. Phosphorous slag powder is beneficial to the development of long-term strength of concrete.