Preparing spherical powders from high-performance engineering plastics is usually challenging due to their high characteristic temperatures and excellent chemical resistance. This study reported an innovative liquid-phase microwave heating method for rapid spheronization of amorphous polyetherimide (PEI) powder and explored the processability of resultant powder in laser powder bed fusion (PBF-LB). The results indicated that the microwave process conditions, including the type of plasticizer liquids and microwave time, significantly influenced the spheronization effect and physicochemical properties of the powder. The plasticizing effect of the liquid largely reduced the glass transition temperature and melt viscosity of PEI particles, promoting their spheronization process. Spheronization enhanced the PBF-LB processability of PEI powder and reduced structural defects in parts, thereby leading to improved mechanical properties. Furthermore, its valuable flame-retardant properties were largely preserved. These findings showed the power of microwave spheronization in plasticizer liquids for advancing the application of optimized high-performance engineering plastic powders in the field of PBF-LB.