In order to explore the effect of the addition of fly ash on crystallization performance of molten modified slag in the preparation progress of blast-furnace-slag-based slag fiber, theoretical crystallization temperature and species and contents of precipitated phases at different addition of fly ash in cooling process of modified slag were simulated by FactSage thermodynamic software and crystallization behavior of molten modified slag with different addition of fly ash was analyzed by XRD and SEM. The results indicated that theoretical crystallization temperature of molten modified slag gradually decreased with the increase of addition of fly ash. Crystallization behavior did not occur staying as glass phase at 1000℃ when the addition of fly ash exceed 15%. It means that the crystallization of molten slag was not the restricted condition of slag fiber forming. Based on the actual working conditions of the fibrillation process of quenched and tempered blast furnace slag, the results of XRD analysis are more referential.