Careful calibration of system geometric position and camera errors is crucial to achieve high testing accuracy for large-scale aspherical surfaces. We propose a geometrical optical calibration method for increasing the overall accuracy of a large-scale reverse Hartmann test system to realize the parabolic reflector measurement. By using a flat crystal, a mask with uniform holes and an external pinhole aperture in front of the camera, camera lens distortions and keystone distortion, as well as system geometry correspondence between LCD screen, camera, and reflecting point on the test surface, are determined accurately. We experimentally demonstrated that the proposed calibration method achieves high slope detection accuracy for a large-scale parabolic reflector: tens of microradians within 1620(H)×850(V)mm2.