Starch and non-starch polysaccharides (NSPs) are two important nutrients, which cause different blood glucose responses after ingestion. To develop novel starchy-based food with low glycemic index values, the introduction of NSPs into starchy system is one of the promising methods. However, in our previous study, NSPs from jicama root (named as PEP-6P) unexpectedly reduced the digestive resistance of jicama starch (PES). To unravel the underlying mechanisms, PEP-6P was treated by three amylolytic enzyme systems. Results showed that PEP-6P could be rapidly digested by α-amylase, indicating the possible existence of α-(1→4)-glucosyl bonds. Further structural analysis confirmed this deduction, and a proposed galactoglucan main chain of PEP-6P was deduced based on the methylation-GC/MS and NMR data. Conformation analysis suggested that PEP-6P presented spherical and aggregated structures in water and 0.1M NaNO₃. These findings revealed that the unexpected pro-digestive effect of jicama NSPs on starch is attributable to the presence of α-(1→4)-glucosyl linkages in the galactoglucan backbone, providing a molecular basis for formulating low-GI jicama-based foods.