A new green synergistic extraction agent (di-2-ethylhexyl phosphoric acid (D2EHPA)-decanol) was developed to recycle Ni(II) from nickel plating effluent. The results show that the D2EHPA-decanol mixture carrier provided a significant synergistic effect in the process. When sulfonated kerosene or sunflower seed oil was used as a diluent, the single extraction efficiency of Ni(II) reached 97.8% and 97.1%, respectively. After three-times extraction, the Ni(II) concentration in the nickel plating effluent decreased to below 1 mg/L. Additionally, in the stripping stage, the stripping efficiency of Ni(II) in the D2EHPA-decanol mixed carrier was larger than that of D2EHPA. Mechanism studies indicate that the polar groups (—OH) from decanol initially break the hydrogen bonds in the D2EHPA dimer structure. Afterwards, van der Waals forces shorten the bond length of P—O to form electron-rich centers of O atoms. Finally, a new P—O—Ni structure is formed through interactions between P—O bond and nickel ions under electrostatic forces.