Based on the principle of hollow ring achieved by conical refraction effect in biaxial crystal, the formula of phase difference distribution between every point and the minimal point in the hollow ring are deduced, and the regularity of phase difference distribution is obtained by means of simulations. The continuity in the circumference of polarization direction variation of each point in the hollow ring is proved. In the experiment, un-polarized He-Ne laser is adopted to perpendicular radiate KNbO3 crystal, then polarization characteristic of hollow beams achieved by conical refraction effect in biaxial crystal is tested. The result of theoretical analysis agrees well with experiment.