The linear astigmatism balance condition of the off-axis three-mirror optical system was deduced, on that base the off-axis three-mirror reflective system for astigmatism correction with large relative aperture and large field of view was proposed by tilting the optical axis. The field of view is 5°×5°, relative aperture is 1/3.1, entrance pupil diameter is 250 mm and spectral range is 400~2 500 nm. The proposed system was compared with the off-axis three-mirror reflective system of coaxial quadratic aspheric surface and the off-axis three-mirror reflective system of coaxial high-order even aspheric surface with the same optical parameters. The results show that the optical transfer function of off-axis three-mirror reflective system with tilted optical axis is greater than 0.73 at 17 lp/mm in all field of view. It uses conical surfaces and shows obvious advantages in imaging quality. In addition, the processing and adjustment of off-axis three-mirror reflective system with tilted optical axis are analyzed, the tolerance is relatively loose and the structure can be realized.