Dextran-containing sourdoughs have been widely employed in baking, obtaining products with higher quality instead of adding additives. This study investigated effects of dextran of a high weight-average molecular weight (Mw) and linear structure, sourdough and their combination on wheat bread quality. The underlying mechanism was explored from properties of pasting, amylopectin retrogradation and water distributions of starch systems. Bread characteristic analyses showed that both dextran and weak acidification due to addition of sourdough improved bread quality, especially crumb softness. Dextran decreased moisture migration due to high water-binding capacity. Enhancements of amylose aggregation by weak acidification contributed to retardation of amylopectin recrystallization. A synergy existed between dextran and weak acidification. Dough rheology demonstrated that weak acidification was fundamental for dextran-induced positive dough viscoelasticity. This study indicates a combined anti-firming performance of high Mw dextran and weak acidification, possibly being a promising strategy to prolong shelf life of wheat bread.