In this study, we discussed the change in properties of a polymer alloy consists of polyetherimide and polyesters while adding a reactive compatibilizer. Moreover, we spun the polymer alloy to have fiber and then tested the flame retardant property of it. The evaluation started from computer simulation and then real compounding, followed by properties measurements. It showed that the compatibilizer would change aggregation scale of different polymers, which agree with the phenomenon in the simulation. Also, the thermal properties of the polymer alloy and crystallization behavior changed with the addition of compatibilizer.