A columnar thin film was fabricated with oblique angle deposition and applied in a prism coupling system as a polarization converter. The transmission, reflection and propagation for the wave reflected in one-round trip from the biaxial film were analyzed. Constructive interference occurred for polarization coupling to achieve polarization conversion. Birefringence of the biaxial thin film provided a sophisticated phase compensation as the incident angle varied over a large range to have a wide-angle polarization conversion. For broadband illumination applications, high polarization conversion efficiency was achieved over visible wavelengths.