A rheology modifier (styrene-acrylonitrile copolymer, SAN) and a flame retardant (resorcinol bis(2, 6-dixylyl phosphate), RXP) were incorporated in polycarbonate (PC)/chopped carbon fiber (CCF)/graphene nanoplatelet (GNP) system to prepare PC-based composites (PC/SANx@(CCF+GNP), rPC/SANx@(CCF+GNP)i) by a twin-screw extruder. The rheological properties, thermal stability, flame retardancy and mechanical properties of composites were evaluated. Increasing shear rate and temperature induced obvious reduction of viscosity of all composites. By comparison, PC/SAN5@(CCF+GNP) possessed a greater viscous flow activation energy (Eη), showing higher temperature sensibility of viscosity. While PC/SAN0@(CCF+GNP) and rPC/SAN5@(CCF+GNP)1.5 had lower Eη values and exhibited different sensitivity of shearing force. Besides, SAN and RXP had obvious influences on the thermal stability of composites. Meanwhile, PC/SAN composites displayed improved flame retardancy from 31.9% of limiting oxygen index (LOI) and V-1 rating for PC/SAN composites to 37.9% of LOI and V-0 rating for composites containing 2.5% RXP. Additionally, mechanical strengths and moduli of PC/SAN composites were increased by SAN and RXP without impairing toughness. These results were attributed to matrix enhancement of SAN, the lubrication effect of lower molecular weight RXP and the enhancement of the interactions between matrix and fillers.