The health effects of dietary fiber have been widely concerned, which are closely related to physicochemical properties. This study focused on soluble dietary fiber of Flammulina velutipes (FDF), evaluated the effects of modifications on structural characterization, the physicochemical properties and the heavy metal adsorption characteristics, and further clarified underlying mechanisms on Pb2+ adsorption behavior of FDFs. The results showed the modifications of extrusion and cellulase improved the yield of FDFs, increased the release of active groups and enhanced the adsorption ability in vitro. Besides, Pb2+ adsorption altered porous structure and led to the presence of carboxylate. It was a spontaneous endothermic reaction and can be fitted by the pseudo-second-order kinetic equation. The Freundlich equation was suitable to describe the adsorption isotherm. These results highlighted potential applications of the dietary fiber modification and laid the theoretical foundation for the modification processing of F. velutipes and protection from food-derived heavy metal toxicity.