Abstract: Based on the aberration sensitivity of structural parameter of offaxis threemirror anastigmat (TMA), the fabrication sequence of the primary mirror, the secondary mirror and the tertiary mirror is selected. The method is to determine the exact fabrication tolerance of each mirror through the optimization step by step. All three mirrors′ structural parameters are measured after fabrication, and these parameters are used to determine the assembly tolerance through optimization with optic system design software. By taking fabrication, alignment and design process into consideration, the fabrication and assembly tolerances are released to the maximal degree while the system specification is still achieved.