Currently, flexible foldable screens have gained prominence in the field of consumer electronics, thanks to the maturity of organic light emitting diodes (OLEDs) and flexible thin film electronics. Flexible screens are composed of multiple layers structures, such as the base layer, the protective layer and the display layer. The display layer with limited deformation ability is expected to be in the neutral layer to minimize stress during folding. Existing research mostly concentrates on the theoretical simulation or experimental research on the deformation of the screen surface, which can not accurately reflect the actual deformation of the display layer during bending. It is essential to study the deformation field of the display layer under bending conditions. This paper proposes a method for measuring the bending deformation of flexible screens based on 3D-Digital Image Correlation (3D-DIC). The proposed method combines displaying speckles on the screen with spraying speckles outside the screen, enabling concurrent measurement of both the display layer and protective layer of the flexible screen. The experimental results show that, at a certain bending angle, the display layer of the screen experiences a smaller compressive strain compared to the surface protective layer. When local debonding occurs, the display layer is affected by a larger compressive strain, which can lead to screen damage. This paper provides new experimental tools and ideas for non-destructive testing, design parameters optimization, and manufacturing processes improvement for foldable screens.