The mode distributions of unstab le resonators with a phaseunifying output coupler are calculated by Prony met hod, and the relations between mode discrimination, far-field divergence angle and some system parameters are analyzed. The results demonstrate that an output beam with diffraction limit can be obtained at small magnification (M=1.3) by this novel resonator. Compared with conventional unstable resonator,it can be better used to high power chemical oxygeniodine lasers (COIL).