An optical design method for large aperture zoom optical system was introduced.The optical system was divided into two segments called front optical system and back optical system.The front optical system used a reflective system to meet the request of large aperture and the other one used a converse zoom objective.The two segments were assembled at the first imaging of the optical system to ensure the imaging quality of the optical system through optimizing the converse zoom objective of back optical system.This method is proved feasible from a design example,it also shows that designed system is simple and compact and easy to fucus for zoom lens with large aperture.