We investigate the aberration properties of the conformal optical system with decentered and tilted elements by vector aberration theory. By decentering and tilting the window and corrector of the system, two elements are effectively used together in a particular manner by aberration compensation to achieve off-axis imaging. A conceptual design is performed with a half-field of 2?, the F# of 4, and the wavelength ranging of 3700~4800 nm. The imaging quality can reach the optical diffraction limit and satisfy corresponding requirements.