An alignment scheme based on the principal component analysis is proposed. The sensitivity matrix of ideal off-axis three-mirror anastigmatic system is processed to obtain weights of system misalignment to residual aberration sensitivity. The reasonable alignment scheme is formulated by removing adjustments with low sensitivities. Based on the principle of computer aided alignment technology of the reverse optimization method, the simulation alignment and the alignment scheme are verified. The results show that with the proposed alignment scheme, the alignment accuracy is up to 10 -7 magnitude, and the adjustment amount is reduced from 11 to 6. The alignment difficulty is reduced, and the stability of optical and mechanical structure is improved.