Deep eutectic solvents (DESs) offer attractive options for the “green” dissolution of cellulose. However, the protic hydroxyl group causes weak dissolving ability of DESs, requiring the substitution of hydroxyl groups in the cation. In this study, a novel allyl-functionalized DES was synthesized and characterized, and its possible effect on improved dissolution of cellulose was investigated. The DES was synthesized by a eutectic mixture of allyl triethyl ammonium chloride ([ATEAm]Cl) and oxalic acid (Oxa) at a molar ratio of 1:1 and a freezing point of 49 °C. The [ATEAm]Cl-Oxa exhibited high polarity (56.40 kcal/mol), dipolarity/polarizability effects (1.10), hydrogen-bond donating acidity (0.41), hydrogen-bond basicity (0.89), and low viscosity (76 cP at 120 °C) owing to the π-π conjugative effect induced by the allyl group. The correlation between temperature and viscosity on the [ATEAm]Cl-Oxa fit the Arrhenius equation well. The [ATEAm]Cl-Oxa showed low pseudo activation energy for viscous flow (44.56 kJ/mol). The improved properties of the [ATEAm]Cl-Oxa noticeably promoted the solubility (6.48 wt.%) of cellulose.