An optical system is designed to shape the rectangular beam which emitted by direct high power laser diode stack for laser material processing. Beam parameter product theory and inverted Kepler telescope system principle are applied to analyze and design the optical system, the fast and slow axis beams of LD stack are focused in same focal plane. Firstly, two LD stacks with different wavelength are collimated by micro-lens array respectively. Then they are combined by dichroic beam splitter. Because of the divergence angle in slow axis direction is still big, the inverted Kepler telescope system is applied to expand and collimate the slow axis beam. Finally, the fast and slow axis beams are focused simultaneously. The whole optical path is simulated by ZEMAX software in the non-sequence mode and the simulated focused spot size and optical power density is obtained after ray tracing. Based on the theoretical calculation and software simulation, we perform the experiment and obtain a focused spot whose size is 2.0mm×5.0mm with 300mm focal length and 5 kilowatts power. The optical power density can reach to dozens of thousands of watts level. We discuss the influence factors of the focused spot size, and analyze the merit and demerit of the optical system. We also point out the improving orientation in the future. The laser beam after shaping by this optical system can be directly applied to laser cladding because of its long focal length and high optical power density.