Slag fiber modified(SFM) with acidity coefficient of 1.6 were treated with acid solution at pH of 5.0, de ionized water at pH of 6.8, and cement hydration solution at pH of 13.6 respectively for determination of durability of SFM. The results show that tensile strengths of single slag fiber are increasingly enhanced with acid solution treatment after 1day; tensile behaviors of fibers are not affected by de ionized water; however significant decrease of the tensile strengths under cement hydration solution are detected in different periods and tensile strengths disappear after 56days duo to the fibers split into small pieces. No apparent change in chemical composition and microscopic shape of the fibers result from de ionized water and weak acid solution treatment. New hydration products are observed on the surface of SFM after alkali solution treatment with the result that elements of calcium, aluminum and magnesium are obviously extracted from SFM, and chemical composition on the surface of SFM is heterogeneous. A small quantity of scawtite crystal is formed.