A polarization converter based on cascaded transverse Pockels effect is presented. It consists of three lithium niobate crystals with external electric fields perpendicular to the light propagating direction. The refractive index ellipsoid theory and wave coupling theory for Pockels effect are employed to study the process of light propagating along the crystals. The study demonstrates that the polarization converter can transform an arbitrary polarized light to any desired polarized light by varying the three external electric fields. The examples to transform an arbitrary light to a linearly polarized light,a circularly polarized light and a desired elliptically polarized light are given.