Granulated　blast　furnace　slag　(GBFS)　is　a　by-product　of　manufacturing　iron.　Samples　of　GBFS　with　different　ratio　of　SiO2/Al2O3　were　prepared　by　simulating　the　actual　process　of　GBFS　in　laboratory.　This　study　investigated　the　influence　of　SiO2/Al2O3　content　on　structure　and　hydration　activity　of　GBFS　which　were　characterized　by　X-ray　fluorescence　(XRF),　powder　X-ray　diffraction　(XRD),　infrared　radiation　(IR)　and　mechanical　testing.　It　is　found　that　the　vitreous　content　of　each　sample　is　above　97%　and　the　hydration　activity　indexes　of　7d　and　28d　of　samples　significantly　decrease　with　the　increase　of　SiO2/Al2O3　ratio.　The　IR　characteristic　absorption　spectrum　shows　that　the　silicates　mainly　exist　in　[SiO4]-tetrahedra　and　the　aluminum　atoms　are　in　different　coordination　states　and　the　bonding　strengths　rise　with　the　increase　of　SiO2/Al2O3　ratio.　©　(2015)　Trans　Tech　Publications,　Switzerland.