The depolarization theory and numerical calculation model for a dual Babinet depolarizer applied to rectangular pupils are built, which makes up the deficiency of the current design theories only suitable for circular pupils. A type of dual Babinet depolarizer with high reliability which can be applied to rectangular pupils is designed. On the basis of the optimal structural parameters，the error requirements of the refractive index and the central thickness difference of crystals are analyzed. The results show that the maximum residual polarization degree of the output light from the depolarizer is less than 2% and the maximum separation distance for the four images is 27 μm. The design requirements of polarization sensitivity and image quality are satisfied.