Many important information of a crystal is included in the convergent polarized light interference pattern, and can be completely utilized by numerical analysis of the pattern. Based on the relationship of wave vectors on crystal interface, accurate formulas calculating phase difference between two refracted waves are deduced. Changes of light polarization in each interface are analyzed and the method to calculate total amplitude in convergent polarized light interference is proposed. The conoscopic interference patterns of crystals are calculated for any orientation biaxial crystal, which include information of isochromatic curve and isogyre. The interference patterns are simulated by digital image and the changes of the patterns are discussed. Experiment of the conoscopic interference is completed for four pieces of KTP crystals with different orientation. Comparison of the experimental patterns and theoretical patterns can judge which Sellmeier's equation is best.