Herein, we propose a new method for promoting the degradation of a perylene diimide (PDI) dye, through a Fenton reaction with cucurbit[8]uril (CB[8]) as a supramolecular catalyst. The CB[8] can encapsulate the hydrophobic moiety of the PDI dye and inhibit its aggregation in aqueous solutions, thus increasing the collision frequency between the PDI and oxidants to accelerate the reaction. As a result, the encapsulated PDI molecule is preferentially degraded, followed by freeing the cavity of CB[8] and enabling it to encapsulate another PDI molecule to realize a catalytic cycle. Hence, a catalytic amount of CB[8] is sufficient to accelerate the the Fenton degradation. It is anticipated that this work will extend the realm of supramolecular catalysis systems and enrich the field of degradation of polycyclic aromatic dyes.