The key design of off-axial Three-mirror Anastigmat(TMA) systems was summarized, and an improved design of the off-axial TMA optical systems was proposed with a new optical structure. By taking two designed systems for examples, the optical elements of the TMA system were reasonably arranged, the secondary mirror was designed as a spherical surface, and the primary and the third mirrors were optimized with high-order aspheric coefficients. Obtained results show that the Modulation Transform Function(MTF) curve of the designed system approaches to the optical diffraction limit with an improved value of 0.6 at the spatial frequency of 50 pl/mm. Compared with the common off-axial TMA optical system, the Strehl ratio can be increased from 0.91 to 0.93. The surface tolerance is relaxed from λ/50 to λ/40, and the assembling tolerances of the primary mirror, second mirror and the third mirror can be relaxed by a factor of 4. The improved design makes the manufacture and assembly of the off-axial TMA optical system much easier, and helps the system to achieve excellent performance in the level of optical diffraction limit. These are beneficial to the popularization of off-axial TMA optical systems.