The popular nonfullerene acceptors readily react with the amine group‐containing low‐work‐function interfacial layer (polyethylenimine [PEI] or polyethylenimine ethoxylated [PEIE]), leading a severe “S” shape in the current density–voltage (J–V) characteristics of the nonfullerene organic solar cells (OSCs). Herein, a novel strategy is proposed to passivate this detrimental interfacial chemical reaction. Phenol (Ph) is anchored on the surface of PEI to form a thin and dense self‐assembly layer, which can avoid a direct contact between the PEI and active layer. Moreover, the formed PEI–Ph bilayer cathode interface layer (CIL) can still low the work function of the bottom electrode. As a result, significant enhancement of the device performance is obtained for the PEI–Ph CIL‐based nonfullerene OSCs and the “S” shapes in the J–V characteristics are removed. After Ph anchoring on the surface of PEI, the power conversion efficiencies are increased from 2.46% to 11.97% for the PTB7‐Th:IEICO‐4F active layer system and from 6.09% to 16.34% for the PM6:Y6 system, respectively. More importantly, the PEI–Ph CIL‐based device displays a superior long‐term and illumination stability.