To meet the application needs of wide field and wide spectrum of earth environment remote sensing, a center without obstructing is realized by means of field off-axis on the basis of coaxial three-mirror optical systems. An optical system with a Cook three-mirror structure is designed. The optical system has its focal length of 120 mm, F number of 3.5, operating wavelength of 0.4 to 1.65 μm and pixel size of 7.5 μm. it realizes the 30°×4° large field of view without any free-form surfaces. In the optical system, the primary mirror is a 6 times hyperboloid; the second mirror is a secondary flat ellipse and the third mirror is a 4 times flat ellipse. The system has its Modulation Transfer Function (MTF) greater than 0.6 at the Nyquist freaquency in the whole field of view, which is close to the diffraction limit. Its RMS dispersion spot diameter is less than the pixel size of the detector and its distortion is less than 2.5%. These results show that the system has excellent imaging performance.