This study explored the effects of a new polydextrose addition on the sensory score, textural parameters, and microstructure of steamed bread, as well as the pasting, thermal, and thermal–mechanical properties of high-gluten wheat flours. Compared with the control sample, 3–10% polydextrose addition significantly increased the hardness, adhesiveness, gumminess, and chewiness of steamed bread, but other textural parameters like springiness, cohesiveness, and resilence remained basically the same. Additionally, 3–10% polydextrose addition significantly reduced the specific volume and width/height ratio of steamed bread but increased the brightness index, yellowish color, and color difference, improved the internal structure, and maintained the other sensory parameters and total score. Polydextrose addition decreased the peak, trough, final, breakdown, and setback viscosity of the pasting of wheat flour suspension solutions but increased the pasting temperature. Polydextrose addition significantly reduced the enthalpy of gelatinization and the aging rate of flour paste but increased the peak temperature of gelatinization. Polydextrose addition increased the crystalline regions of starch, the interaction between protein and starch, and the β-sheet percentage of wheat dough without yeast and that of steamed bread. Amorphous regions of starch were increased in dough through the addition of polydextrose but decreased in steamed bread. In addition, 3–10% polydextrose addition decreased the number of random coils, α-helixes, and β-turns in the dough, but 3–7% polydextrose addition maintained or increased these conformations in steamed bread, while 10% polydextrose decreased them. In steamed bread, as a hydrogel, 5–7% polydextrose addition resulted in the formation of a continuous three-dimensional network structure with certain adhesiveness and elasticity, with increases in the porosity and gas-holding capacity of the product. Moreover,10% polydextrose addition further increased the viscosity, softness, and looseness of the steamed bread, with smaller and more numerous holes and indistinct boundaries between starch granules. These results indicate that 3–10% polydextrose addition increases the softness and looseness of steamed bread by improving the gluten network structure.