With continuous increases in the apertures of ground-based telescopes， the size and weight of the lens barrel structure is becoming greater and greater， and the spatial position change of the primary and secondary mirrors caused by the finite rigidity of the structure and the change of the gravity field is becoming increasingly obvious. The accurate measurement and correction of the deflection of the primary and secondary mirrors of a large-aperture telescope is a necessary prerequisite to ensure its imaging quality and pointing accuracy. Based on the structural characteristics of a large-aperture telescope， the factors affecting the position and attitude of the primary and secondary mirrors are analyzed， and an optical measurement method based on the internal focusing collimator is proposed. A telescope with a distance of 5.5 m between the primary and secondary mirrors is measured as an example. The maximum radial offset in the horizontal direction is 196 μm， the maximum radial offset in the vertical direction is 16 μm； the maximum angular deflection in the horizontal direction is 2.6″， and the maximum angular deflection in the vertical direction is 12.8″. Finally， the measurement uncertainty involved in this method is analyzed. The combined standard uncertainty of measuring radial offset is 9 μm， and the combined standard uncertainty of measuring angular deflection is 0.65″， meeting the alignment accuracy of large-aperpure telescopes.