In order to explore the mode of corner cube resonator, taking the cone plane mirror cavity as an example, the corner cube prism is equivalent to a diffraction grating. Considering the influence of the diffraction effect of the corner pyramid width in the resonator and the additional phase distribution caused by the dihedral angle error on the laser mode of the resonator, a theoretical analysis model for solving the intrinsic mode is established based on the theory of the optical resonant cavity. The fast Fourier transform method is adopted to simulate the distribution of the laser oscillation modes of the passive corner cube cavity in different cavity length, apex widths and dihedral angle error. When the cavity length is 30 cm, the width of the corner cube is less than 75 μm, and the angle error of the two sides is between -10'and 5', the complete circular distribution output mode of the light spot can be realized, and the beam quality is good. When the width of the edge is not less than 0.4 mm and the dihedral angle error is between -40'and 10', the spot is a transverse mode of TEM₀₃ and the light field is a six-lobe distribution. When the width of the corner cone mirror is 0.4 mm, the error of the dihedral angle is 3' and the cavity length increases in the range of 30~90 cm, the laser mode of the resonator is converted from TEM₀₃ to TEM₁₀.