There is considerable interest in cellulose solubilisation in deep eutectic solvents (DESs) as a new emerging class of green solvents. However, the experimental data have indicated that cellulose is almost insoluble in most of DESs. Carboxylic acid-based DESs are a category of DESs that exhibit a modest ability to solubilise cellulose. However, the molecular-level reasons why carboxylic acid-based DESs can dissolve cellulose are unknown. Such information is crucial in developing DESs with enhanced cellulose solubilising ability. Here, we use molecular dynamics simulations to explore the molecular mechanism of cellulose solubility in oxaline (choline chloride/oxalic acid) as a typical carboxylic acid-based DES and cellulose insolubility in reline (choline chloride/urea) as the most famous DES. It was revealed that oxalic acid possesses a greater ability to weaken the interaction between the chloride anion and the choline cation when compared to other urea in reline. Self-interactions of hydrogen bond donors are also highly attenuated in oxaline than in reline. These results explain the observed more significant melting point depression of oxaline in comparison to reline. We depict a molecular mechanism indicating how oxalic acid synergically works with chloride ion to dissolve cellulose and why urea cannot play such a role in reline.