A annular output unstable resonator with 90° beam rotation suitable for high-power supersonic chemical oxgen-iodine lasers (COIL) is given, intensity distribution of lowest-order mode with inhom ogeneous distribution of small signal gain is calculated by FFT method.The prop erties of forward mode and reverse mode of this resonator at magnification M＝1.21 and 1.3 are studied by copper vapor laser experiments, a good experimental result is obtained．