This paper introduced an optical design method for large aperture zoom optical system. The optical system divided into two segments which called front and back with this method. The front optical system used reflect system to meet the request of large aperture and the back optical system used the converse zoom objective. The two segments would be assembled at the first imaging of the optical system. It would assure the imaging quality of the optical system through optimized the converse zoom objective of the back. This method had been proved feasible from the design results through an example. This method also could assured the configuration simple and compact and solved the matter which hard to design zoom lens with the large aperture.