Abstract: A series of carboxylic acid-functionalized deep eutectic solvents (DES) were constructed by a natural organic dicarboxylic acid, oxalic acid (OA), as the hydrogen bond donor and the polyethylene glycol (PEG) with different polymerization degrees as the hydrogen bond acceptors, which are used in the hydration of α-pinene to produce α-terpineol. Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (¹H NMR), and thermogravimetric analysis (TGA) were used to prove the hydrogen bonding between OA and PEG. It is found that the presence of PEG has a less impact on the acid strength of DES. However, an increase in both molecular weight and dosage of PEG results in a decrease in total acidity and catalytic activity. Among them, OA/0.6PEG200, a DES catalyst prepared by PEG with the smallest molecular weight, exhibits a favorable catalytic performance. Under an optimal reaction condition with 0.03 mol of DES (based on OA), 0.06 mol of α-pinene, 0.3 mol of water, at 75 °C for 8 h, an α-pinene conversion of 81.5% and an α-terpineol selectivity of 51.2% are obtained. The catalyst phase can be separated by refrigerating overnight after reaction and reused directly with relatively stable catalytic performance. Thus, OA/0.6PEG200, as a DES catalyst prepared by a simple and highly atom economical process, will offer a clean catalytic route for the one-step production of α-terpineol.