This study investigated the impact of in situ exopolysaccharides (EPS) on the properties of dough and quality of Chinese steamed bread (CSB). Sourdoughs were fermented with dextran-producing Weissella confusa QS813, with and without sucrose addition at 20 °C and 35 °C, respectively. The highest EPS content and lowest titratable acidity value were obtained in sucrose-enriched sourdough fermented at 20 °C (WS20+). Addition of WS20 + significantly improved proofing performance and induced glutenin macropolymer (GMP) particle agglomeration of dough. Microstructure results revealed a more continuous, connected network in WS20 + dough and better crumb structure in the corresponding CSB. The quality of CSB was enhanced upon addition of WS20+. Low EPS yield in sourdough without sucrose negatively affected dough and CSB quality. Increased acidification in sucrose-enriched sourdough fermented at 35 °C masked the beneficial effects of EPS. Therefore, both high EPS generation and mild acidification contributed to the beneficial effect of W. confusa sourdough on CSB quality.