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Electric-field-induced second harmonic generation (EFISH),a third-order nonlinear process,arises from the interaction between the electric field of an exteal bias and that of two incident photons.EFISH can be used to dynamically control the nonlinear optical response of materials and is therefore promising for active nonlinear devices.However,it has been challenging to achieve a strong modulation with EFISH in conventional nonlinear materials.Here,we report a large tunability of an EFISH signal from a subwavelength-thick polymer film sandwiched between a transparent electrode and a metallic mirror.By exploiting the band-edge-enhanced third-order nonlinear susceptibility of the organic conjugated polymer,we successfully demonstrate a gigantic EFISH effect with a modulation ratio up to 422％V-1 at a pumping wavelength of 840 nm.The band-edge-enhanced EFISH opens new avenues for modulating the intensity of SHG signals and for controlling nonlinear electro-optic interactions in nanophotonic devices.