In this study, a rapid and accurate optical axis alignment technique of a reflective beam expander composed of convex and concave off-axis parabolic mirrors was established by using (a) five parallel lasers and (b) shear interferometry. The five parallel lasers are placed on the top, bottom, left, and right sides, and center of the disc fixture, forming a circular symmetrical distribution and parallel to each other. Since the convex and concave off-axis parabolic mirrors are both confocal systems, the central laser of the five parallel laser beams is incident along the optical axis of the reflective beam expander, and the surrounding four beams diverge outward after the first reflection of the convex off-axis parabolic mirror, and these diverging laser beams converge inward after the second reflection of the concave off-axis parabolic mirror, so that the four output beams are parallel to each other, maintaining a circular distribution and a large diameter, but the divergence angle is relatively small relative to the incidence. The optical plate with excellent parallelism is placed in front of the collimated beam at an angle of 45°. The reflected beams on both sides of the optical plate are expected to interfere. In this study, this shear interferometry method was used to measure the collimation of a parallel beam after passing through a reflective beam expander. In this experiment, a HeNe laser was used as the light source, and its wavelength and beam diameter were 0.6328 μm and 3 mm, respectively. The test sample is a beam expander with a magnification of 2× and an incident beam diameter of 3 mm. After 30 repeated tests, the experimental results showed that the beam diameter increased to 5.78 mm and that the magnification accuracy was 96.3%.