In order to meet the needs of the wide field-of-view (FOV) imaging spectrometer development, the problem about optical design of telecentric, wide FOV, large relative aperture, off-axis three-mirror system is studied, and the expressions of initial configuration parameters and the third-order aberrations for flat field, telecentric, three-mirror system are educed. With the specific requirements of 0.4~2.5 μm spectral range, F number is 4, 720 mm focal length, 10° FOV, a flat field, telecentric, off-axis three-mirror imaging system is designed, in which the secondary mirror is convex spheric mirror, the primary mirror and the third mirror are concave conicoid and coplanar, and all the three mirrors are coaxial. The design result indicates that the imaging quality of the designed system approaches the diffraction limit at full FOV in the required wave band.