Nowadays, flexible substrates, as a part of flexible electronics, receive great attention by its wide application. Particularly, organic light emitting diodes and photovoltaics place high demands on the optical transmittance of flexible substrates. In this paper, direct fluorination is selected to treat commercial poly(ethylene terephthalate) (PET) and polyimide (PI) films. A suitable gradient distribution of fluorine atoms in the fluorinated layer were obtained in the vertical direction of the PET film by optimizing the conditions of direct fluorination. In this way, the refractive index was reduced by the introduction of the fluorine atoms, and the fluorine atoms with gradient distribution on the film's vertical direction led to the gradient decrease of the refractive index. Given this gradient- refractive index, a high broadband and omnidirectional transmittance of > 93.0% was achieved by such a non-nanostructured surface over the entire optical wavelength (380-2200 nm), while the transmittance still remains relatively high (> 85.0%) when the incident angle is increased from 0 degrees to 60 degrees. Moreover, this cost-effective and easy method presents its universality with the treatment of PI film, which also achieves high broadband and omnidirectional transmittance by direct fluorination.