The effects of choline chloride/oxalic acid deep eutectic solvents (ChCl/OA DES) as a green and effective promoter assisting the synthesis of vanadium phosphorus oxide (VPO) catalysts for the selective oxidation of n-butane to maleic anhydride were investigated in detail. A combination of characterizations with the performance was considered to understand the essential effects of DES. DES play the role of a crystal induced agent and structural modifier, facilitating the formation of a single-crystal structure on the surface of precursor; correspondingly, topological transformation to the single-crystal active phase under the activation conditions accompany the decomposition of DES. It is suggested that ChCl/OA DES can interact with V2O5 and form a new vanadium complex, which affects the reaction between V2O5 and H3PO4. Meanwhile, the ChCl/OA DES could regulate the surface chemical state and redox characteristic, resulting in the enhancement on the catalytic performance of VPO.