The invention provides a large-aperture telescope secondary mirror attitude in-situ adjustment method. A collimator with the exit pupil coinciding with the entrance pupil of a tested telescope is selected, and one part of a middle pupil surface forms a detection pupil surface; wavefront arrives at a wavefront sensor and is then divided to a plurality of sub units by a microlens array, a sub imageformed on a detector target surface offsets in relative to a standard reference sub image, the slope of each subunit of the wavefront is obtained, the wavefront is reconstructed based on the slope toacquire a system aberration condition, the six-dimensional variation of the secondary mirror attitude is thus calculated and fed back to a parallel six-axis displacement platform for attitude adjustment on the secondary mirror, new six-dimensional variation is acquired repeatedly on the basis for adjusting the attitude of the secondary mirror, multiple cyclic iterations are repeated, gradual convergence is carried out, and the purpose of secondary mirror correction is completed. The secondary mirror attitude detection task can be completed through the collimator, the price is low, and the collimator can detect the wavefront change caused by the attitude of the secondary mirror completely by only using the partial aperture of the whole telescope.