Abstract: Based on precise measurements of the surface-shape of cavity mirrors, a cavity model has been achieved by optical simulation software ZEMAX for a folded virtually-confocal unstable resonator employed in COIL. The influences of the surface-shape errors of cavity mirrors on beam quality, and the effects of misalignment of cavity mirrors on the wavefront of output beam, have been analyzed using the model. Beam quality measurements have been performed by using a He-Ne alignment laser in a bare cavity. The measurement results agree with the simulation data, and verify the feasibility of the modeling and simulation method.．