Six kinds of slag with acidity coefficient(AC) from 093 to 160 were fabricated under similar fibration process. Geometry parameters and single filament tensile strength as well as amorphous structure were measured and estimated for the fibers with various chemical compositions. A series of fitted empirical equation to AC were established to correlate AC to length, diameter, the aspect ratio of fiber, partial crystalline temperature, and main peak chemical shift of 29Si and 27Al spectra from solid state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The results show that the length and diameter of the fiber increase with increasing AC and the aspect ratio increases when AC is below 119 and then declines slightly afterwards. There is a clear linearly progressive correlation between single filament tensile strengths and AC. In addition, the crystalline temperature of the fiber rises with increasing AC. Silicon and aluminum atoms exhibit four coordination in network of amorphous phase and greater AC can result in higher polymerization forming more stable fiber structure.