The invention discloses a device and method for adjusting tilt of a measured mirror in matching measurement of a flat sub-aperture. The device comprises a fizeau phase-shifting interferometer, a standard mirror, a semi-transparent and semi-reflection mirror, a measured mirror, a laser autocollimator, a two-dimensional translation stage, a tilt adjusting device, a rotary table, a plane reflection mirror, a computer, a detector and a focusing lens. Levelling is carried out by using the device at the position of a first sub-aperture, and a parallel reference point of the measured mirror and the standard mirror is established, and then the gradient of the measured mirror is calculated based on the reference point at the position of the other sub-apertures, and finally, the posture of the measured mirror is adjusted so that the gradient of the sub-aperture surface is reduced and the accuracy of matching measurement of the flat sub-aperture is increased.