The present disclosure relates to a synthetic vegetable oil and an environmental-friendly flame-retardant hydraulic oil composition including the same, and a preparation method thereof, and provides a method of preparing a glycerin-ester-based lubricating base oil through a catalyst-free and rinsing-free process, a glycerin-ester-based lubricating base oil that is a synthetic vegetable oil obtained by the method, and an environmental-friendly flame-retardant hydraulic oil composition that contains the glycerin-ester-based lubricating base oil and that is capable of reducing electricity consumption by 7 to 8% compared to mineral oil-based lubricating oils due to excellent flame retardancy (with self-extinguishing function) and lubricity.