In order to realize the autonomous guidance, target detection and classification/identification for extended air defence(EAD) and ballistic missile defence(BMD), a concept was presented to integrate both imaging infrared and imaging flash LADAR sensor into a compact seeker setup using the same receiver aperture and the results of optical design in this system was demonstrated. The IR sensor detected the scene, discriminated the reentry vehicle and cued the LADAR to the selected object. The LADAR sensor illuminated the selection with a high power laser pulse and the reflected light will be imaged through the receiver optics onto the focal plane. The image was then sampled in snapshot mode and 3D-information(angle-angle-distance) was generated. It used the surface of dichroitic beam splitter as a dichroic surface, to achieve a laser receiver and infrared imaging of the total diameter, saving system space. Whereafter, the way of calculating the parameters of optical system was illuminated and the results of optical design with CODE V was given out. The optical system designed in this paper can fulfil the requirement of the system scheme owing to its good quality.