For the size control problem of slab laser’s output beam,a two-dimensional continuous zoom beam expender optical system is designed which expander ratio can reach 14~20× in the direction of X and 1.25~1.55× in the direction of Y.The movement rule of the fixed part,zooming part and compensating part are derived through the analysis of the three component zoom lens.The theoretical value is optimized by the ZEMAX,which makes the aberration of the system meet the requirement.Working wavelength of the system is 1064nm,and the size of output beam in a certain range can be expanded to 35~40 mm.The system has some advantages of a simple structure,a short zoom distance and a smooth zoom path.