Abstract: Based on an optical interference phase-contrast theory in the spectral domain, we propose a B-scan method to measure the out-of-plane displacement inside the glass fiber/polymer composite. In order to quantify its performance, composite members are specially designed, in which the polymer layer, air layer and glass layer represented the polymer base, crack and glass fiber in glass fiber/polymer composite. The measurement is executed by two snapping the interferograms before and after the load are applied, therefore, the processing is rapid. The system has the properties as: the field of view is 3.5 mm, the resolution of the contour is ±26.6 μm, the resolution of the out-of-plane displacement is ±0.1 μm, the range of the depth is 2.66 mm. This technology has a bright future in the mechanical properties measurement and non-destructive testing of the glass fiber reinforced resin matrix composite.