Structural changes of pure blast furnace slag fiber and composite slag fibers modified（CSFM） from initial status to treatment by alkali solution has been observed using X ray diffraction（XRD）, solid state magic angle spinning nuclear magnetic resonance（MAS NMR） spectroscopy and scanning electron microscope（SEM） with energy dispersive X ray spectroscopy（EDXS）. It is demonstrated that amorphous phases are primary and polymerization of silica tetrahedral network increases due to the acid degree coefficient（ADC）, with unique presence of tetrahedral aluminum in initial CSFMs. After the fibers immersed in alkali solution for 180 days, both Q1 and Q2 unit play a predominating role from 29Si MAS NMR spectra of higher ADC fibers, along with Q0 remarkably decreased and even disappeared. The results suggest that degree of polymerization in the hydration products increase. New 6 coordinated octahedral peaks at δ=76 site appear in 27Al MAS NMR spectra, compared with initial CSFMs. Areas of the peaks decrease as a result of less depolymerization of tetrahedral aluminum network with increasing ADC. CSFM can effectively improve alkali resistance as its ADC up to 12.