The invention provides an optimized modification method of a flexible substrate. The method comprises the following steps of mixing a polyimide solution and a cellulose nanocrystalline material, acquiring a mixture and coating the mixture on a glass substrate; baking the mixture coated on the glass substrate, removing a solvent in the mixture, carrying out phase separation on the mixture and making the cellulose nanocrystalline material be close to the glass substrate; heating and curing the mixture and acquiring the flexible substrate; and focusing the flexible substrate through using a laser, and stripping the flexible substrate from the glass substrate. In the invention, stripping difficulty of the flexible substrate and the glass substrate can be reduced and a sacrificial layer does not need to be used to carry out stripping.