To address the stability decline of ambient-temperature-storage yogurt (ATSY) caused by gel network damage during secondary pasteurization, this study introduces a novel synergistic ultrasound‑sodium alginate (US-SA) modification strategy and optimizes its process parameters via response surface methodology (US: 192 W, 28 kHz, 8 min; SA: 0.2 %). Compared to the control, synergistic treatment increased the water-holding capacity (WHC) of yogurt by 14.78 %, surpassing the effects of US alone (4.15 %) and SA alone (8.67 %). Synergistic treatment also markedly improved texture, increasing hardness, gumminess, and chewiness by 48.1 %, 62.1 % and 40.6 % relative to the control, while enhancing rheological performance, as indicated by the highest storage (G') and loss (G") moduli and the greatest apparent viscosity. To elucidate the mechanism, ultrasonication alone first fragmented SA via cavitation, cleaving its backbone to reduce molecular weight and alter the M/G ratio, thereby increasing the proportion of rigid G-blocks, strengthening Ca2+ binding in "egg-box" structures, and lowering solution viscosity to aid dispersion. Simultaneously, ultrasound modified milk proteins by decreasing particle size, intensifying surface negative charge and hydrophobicity, and elevating free sulfhydryl content while decreasing disulfide bonds. It also increased β-sheet and α-helix content at the expense of random coils and enhanced intrinsic fluorescence, thereby improving the hydrophobic microenvironment. During fermentation, the low-molecular-weight SA penetrated the refined casein network, forming an interpenetrating matrix with milk proteins via electrostatic interactions and calcium ion bridging, together forming a dense and homogeneous three-dimensional gel network. This study offers a novel strategy for stabilizing ATSY products with minimal use of additives.