The invention provides an automobile headlamp optical system which comprises an LED light source, a reflection part and a lens. The reflection part comprises a first reflecting mirror, a reflecting plate, a baffle and a second reflecting mirror. The first reflecting mirror comprises a first main reflecting mirror body and a first auxiliary reflecting mirror body. The portion, on the rear of the baffle, of the reflecting plate is used for reflecting some light shielded by the baffle to the lens secondarily so that a dipped headlight light type with light and shade cut-off lines can be obtained. The first reflecting mirror is shaped like an ellipsoid. The LED light source and an optical axis are vertically arranged on a first focal point of the ellipsoid. The first reflecting mirror is the upper half portion of the ellipsoid and extends to a second focal point. The baffle is arranged on the portion, located on the second focal point of the first reflecting mirror, of the ellipsoid. The portion, on the top of the baffle, of the reflecting plate reflects some light shielded by the baffle to the lens secondarily so that a dipped headlight light type with light and shade cut-off lines can be obtained; meanwhile, most of the light shielded and filtered out is fully utilized, and the utilization rate of luminous flux is increased.