In order to obtain different diameters of laser beam by using one optical system, a catadiopric beam expander zoom optical system was designed. The beam expanding ratio was changed from 55 to 155 continuously, which was greater than before, and could expand the application areas. The system was composed of two main parts: one afocal zoom system was located in the front could achieve continuous laser beam expanding. Only four sphere lenses were used to design the zoom system. In order to reduce the size of total system, a “Positive-Negative-Positive” form was introduced. The material of variable part of zoom system was analyzed because the beam diameter was so small that the distortion was appeared for the temperature changing by the laser irradiation. The other secondary catadioptric system was used to achieve high ratio beam expanding. A sphere mirror was used to replace an off-axial parabolic mirror, and a meniscus lens was introduced to compensate aberrations. The lens in the secondary system was so important to the performance that it was analyzed, and then the thickness and material of the lens were chosen. The model was built up in CODEV. By optimizing aberrations, the performance of the system reaches diffraction limitation after adding perfect lens. Finally, the software light tools were used to analyze the collimation and the result matches the theoretical design.