A novel compound resonator——a confocal unstable resonator with large aperture coupling which can be applied to the high power lasers is presented in this paper. Design of the resonator is analysed in detail and method of design is provided. The laser modes of the resonator are studied both theoretically and experimentally. The theoretical results coincide with the experimental results. The output power is 0.75 kW with an efficiency of 8.0%, and the divergent angle in the far field is 1.73 mrad.