Herein, an efficient full-spectrum responsive p-Ag₂S/n-PDI (perylenediimide) heterojunction was successfully constructed. The self-assembled PDI nanostructure was formed via hydrogen bonding and π–π stacking. Ag₂S quantum dots were tightly loaded onto the surface of PDI nanofibers via a two-step electrostatic process. When the mass ratio of Ag₂S to PDI was 1 : 0.6, the p-Ag₂S/n-PDI heterojunction showed optimum photocatalytic properties. The full-spectrum photocatalytic activity of p-Ag₂S/n-PDI was found to be 5.13 and 1.79 times higher than pure PDI for phenol degradation and O₂ evolution, respectively. The results showed that Ag₂S quantum dots promoted the well-organized π–π stacking degree of the self-assembled PDI, which was helpful for the migration of photo-generated electrons along the quasi-one-dimensional π–π stacking of PDI. Simultaneously, Ag₂S quantum dots were found to enhance the light absorption of Ag₂S/PDI. More interestingly, the p-Ag₂S/n-PDI heterojunction exhibited excellent photoelectric properties, indicating more effective separation of carriers, which arose as a result of the built-in electric field between the Ag₂S and PDI. Besides this, the p-Ag₂S/n-PDI heterojunction produces more active species than pure PDI, resulting in a much stronger oxidation ability. This work details some interesting ideas for designing efficient heterojunction photocatalysts that have a supramolecular organic nanostructure.