Melem is the basic structural unit of g-C3N4. When directly utilized as a photocatalyst, the separation efficiency of photogenerated electron-hole pairs is low. In this study, we constructed a Melem-based all-organic heterojunction (PICN) by reacting the -NH2 at the terminal of the Melem unit with the anhydride of PDA. The photocatalytic efficiency for H2O2 production of the heterojunction is 4.2 times that of CN. Introducing -Cl at the bay position of PDI further enhances the photocatalytic H2O2 production efficiency by 1.6 times. This enhancement is attributed to the introduction of -Cl, which not only broadens the UV-visible light absorption range of the sample but also improves the interface electron transfer efficiency between Melem and PDICl. Moreover, the introduction of -Cl can also reduce the overpotential of the O-2 reduction reaction on the surface of the sample. These novel findings may provide insights for the design of organic heterojunction photocatalysts.