Optical windows are important for a wind tunnel to enable observation or imaging of the internal flow-field. In order to reduce the interference to the internal flow and not to modulate the input observation ray field as much as possible, the interior and exterior surfaces of the optical window often adopt complex free-form surface design and must be used in pairs. This requires high accuracy of both the surface form and their relative position. In this paper, Trace-Pro optical software is used for the ray tracing analysis of the wind tunnel observation window defined by discrete points on the inner and outer surfaces. The conclusion is that the collimated beam is also the collimated beam after passing through the single observation window, which means the transmitted wave-front can be well resolved by a standard interferometer. The ray data in Trace-Pro is imported into MATLAB to obtain the modulated wave-front error which is contributed by the difference between the inner and outer surfaces of the monolithic optical window. It hence can be used to guide the corrective machining of the window surface. In addition, the influence of different misalignment on the interferometric test of the window is analyzed. Finally, the method is experimentally demonstrated on an optical free-form window. The surface positioning error is reduced with corrective machining based on the measured transmitted wave-front.