The present invention relates to the field of displays and discloses a polyimide substrate, which is manufactured by reacting lignin, polyimide and a free radical initiator. Because lignin contains various active groups, for example, hydroxyl, carboxyl and aryl, etc., when it is introduced into the polymer structure of polyimide, the maximum absorption peak of the polymer can be made to redshift from less than or equal to 280 nm to less than or equal to 380 nm, so that a certain absorption and screening action may be laid on the light wave during a subsequent Laser Lift Off process, and the substrate and the liquid crystal may be prevented from being damaged during a Laser Lift Off process of the glass base substrate, thereby guaranteeing the display quality of the flexible display.