A novel flame-retardant plasticizer based on castor oil (FRC) was synthesized, and its potential application for polyvinyl chloride (PVC) was evaluated through the study of torque during melt processing, thermogravimetric analysis (TGA), limiting oxygen index instrument, scanning electron microscope (SEM) and mechanical tests. The results suggested that the FRC has a plasticizing effect in blends with PVC. The addition of FRC resulted in reduction of processing torque, good compatibility, improvement in thermal stability and efficient flame retardant. Torque reduction was observed (33.6%), indicating the reduction of viscosity and the improvement of process. The increase (31.9%) of LOI value indicated that incorporation of FRC system had obviously improved the flame retardant property of PVC blends. The excellent flame retardant properties were obtained by forming an isolation layer on the surface of PVC blends in the process of FRC burning. FRC could improve the thermal ability of PVC blends by enhancing the decomposition temperature of PVC. The performance in all properties of PVC products could be obtained by controlling the formula blends. Therefore, castor oil-based flame retardant plasticizers would appear suitable for a wide range of application.