An off-axis three-mirror system (OTS) was designed based on the primary mirror and tertiary mirror (TM) integrated on a single substrate in order to solve the OTS drawbacks, such as the alignment difficulty and the large opto-mechanical weight. Furthermore, an optical freeform surface that can increase the optimizing degrees of freedom (DOF) was applied on the TM in order to achieve a wide field of view (FOV). An example with a focal length of 1200 mm, F-number of 12, and FOV of 10°×4° was given, and the maximum wave front error (WFE) RMS was 0.0126λ, indicating a good imaging quality. The design result shows that the number of alignment DOF was reduced from 12 to 6, and the weight of the mirror support assembly can also be lighter. An XY polynomial, established as an even function of x, was employed as the TM surface, so we obtained an axial symmetrical imaging quality about the x axis, and the axial symmetry aberration performance also brings considerable convenience to alignment and testing for the OTS.