Fiber-coupled diode laser pumping source is one of the key components of high-power fiber laser system. Its performance is significant to the output power of fiber laser system. A 1.8kW fiber-coupled diode laser system is designed by using ZEMAX optical design software. The technologies of high-precision beam collimation, spatial multiplexing, polarization multiplexing, beam expanding, focusing and coupling are used to couple the beams of 42 diode laser bars into a fiber with a core diameter of 200μm and NA 0.22. Every beam emit from diode laser bar is single polarization, and its central wavelength is 976nm @ 55W. The desigh result showed the fiber output power could reach 1800W, and the fiber-coupling efficiency was 78%, the brightness was 37MW/（cm<sup>2</sup>·sr）,corresponding. This fiber-coupled system can be used in fiber pumping, material process and many other areas.