Off-axis three-mirror system(OTMS) has the advantage of high image quality, large field of view and non-obscuration. But drawbacks also exist, such as the alignment difficulty, cost and large optomechanical weight. In order to solve these problems, the design of a novel OTMS was necessary. In this design, the primary mirror and tertiary mirror can be integrated on a single substrate based on third-order aberration theory. An example which has a focal length of 1 200 mm，F-number of 12 and field of view of 10°×1° was given. The mean WFE rms was λ/55, and the maximum WFE rms was λ/22. The result of this design shows that the number of alignment freedoms was reduced from 12 to 6, the weight of the mirror support assembly also can be lighter.