Mixed-linkage (1→3)(1→4)-β-D-glucans (BG) is widely recognized for its health benefits and distinct modification effects in food processing. BG has been approved as a desirable food additive to compensate for the substantial fiber loss that occurs during wheat flour milling. This study systematically evaluated the divergent effects of BG incorporation in two principal wheat-based products: baked bread and steamed bread, employing two distinct barley cultivars as BG sources (ZQ2000BG, derived from Zangqing2000; ZQ25BG, derived from Zangqing25). Then underlying mechanism was elucidated through comprehensive analysis of source-dependent variations in BG physicochemical properties and their distinct interactions with key dough components (water, starch, and protein). The incorporation of BG demonstrated markedly different effects on baked bread versus steamed bread, inducing modest loaf volume expansion in baked bread while causing complete structural collapse in steamed bread. BG interfered with the hydration of starch and proteins, and can bind directly to both components, exhibited significant inhibitory effects on starch pasting, digestion, and formation of disulfide bonds in gluten proteins. BG addition promoted the formation of a looser, finer, and more elastic gluten structure in baked bread dough, but led to thicker and less elastic gluten strands in steamed bread dough. Under the high-humidity, high-pressure conditions in steamed bread heating process, this gluten-disrupting effect becomes exacerbated, ultimately leading to complete structural failure. Furthermore, ZQ2000BG—characterized by higher molecular weight, lower (1→4)/(1→3) linkage ratio, and porous microstructure—demonstrated stronger binding affinity to α-amylase-like proteins, more pronounced starch pasting inhibition, and superior baked bread-making performance compared to ZQ25BG.