The semiconducting properties and applications of three dimensional (3D) covalent organic frameworks (COFs) are greatly hampered because of their long‐ranged non‐conjugated skeletons and relatively unstable linkages. Here, a robust imidazole‐linked fully conjugated 3D covalent organic framework (BUCT‐COF‐7) is synthesized through the one‐pot multicomponent Debus‐Radziszewski reaction of the saddle‐shaped aldehyde‐substituted cyclooctatetrathiophene, pyrene‐4,5,9,10‐tetraone, and ammonium acetate. The semiconducting BUCT‐COF‐7, as a metal‐free catalyst, shows excellent two electron oxygen reduction reaction (ORR) activity in alkaline medium with high hydrogen peroxide (H2O2) selectivity of 83.4 %. When the BUCT‐COF‐7 as cathode catalyst is assembled into the electrolyzer, the devices showed high electrochemical production rate of H2O2 up to 326.9 mmol g−1 h−1. The accumulative amount of H2O2 could totally degrade the dye methylene blue via Fenton reaction for wastewater treatment. This is the first report about intrinsic 3D COFs for efficient electrochemical synthesis of H2O2, revealing the promising applications of fully conjugated 3D COFs in the environment‐related field. A robust imidazole‐linked fully conjugated 3D covalent organic framework was synthesized via the one‐pot Debus‐Radziszewski multicomponent reaction. The material showed excellent two electron oxygen‐reduction activity and when it was used as cathode catalyst and assembled into the electrolyzer, the devices showed high electrochemical production rate of H2O2 up to 326.9 mmol g−1 h−1.