This study investigated the conversion of furfural to 5-hydroxymethylfurfural (HMF) and further to levulinic acid/ester in dimethoxymethane under acidic conditions, with the particular focus on understanding the mechanism for polymer formation. The results showed that furfural could react with dimethoxymethane via electrophilic substitution reaction to form HMF or the ether/acetal of HMF, which were further converted to levulinic acid and methyl levulinate. The polymerization of furfural and the cross-polymerization between dimethoxymethane and the levulinic acid/ester produced were the main side reactions leading to the decreased yields of levulinic acid/ester. Comparing to the other solvent, methanol as the co-solvent helped to alleviate but not totally inhibited the occurrences of the polymerization, as the polymerization reactions via aldol condensation did not eliminate the C=O functionalities. As a consequence, the polymerization reactions continued to proceed. Other co-solvent used such as guaiacol, dimethyl sulfoxide and acetone interfered with the transformation of furfural to HMF or aided the polymerization re-actions. The polymer produced from the reactions between furfural and DMM was different from that produced from levulinic acid/ester. The former had a higher crystallinity, while the latter was more aliphatic. The DRIFTS and TG-MS studies showed that the polymer had the carboxylic group, methyl group and the aliphatic structure in the skeleton. The removal of these functionalities was accompanied by the aromatization of the polymer. The condensation of DMM with levulinic acid/ester was the key reason for the diminished production of levulinic acid/ester.