The in-situ-produced dextrans (DXs) could effectively enhance the viscosity of rice protein (RP) yogurt, but the reason for this improvement has not been elucidated. This study aims to reveal the mechanism underlying the viscosity improvement of RP yogurt by the presence of in-situ DXs. DXs synthesized in RP yogurts under different optimum conditions were purified and fully characterized. RP yogurts were simulated by mixing RP, DXs, lactic acid, and acetic acid according to their real concentrations. The impacts of DXs on the physicochemical properties of RP and the molecular dynamics of the polymers were examined. The minor difference in branching degree (from 5.79 % to 7.08 %) and conformation of DXs could not result in a significant difference in their macromolecular and thermal properties. DXs interacted with RP through hydrogen bonds, leading to a refolding of RP and the formation of a "core-shell" structure. The immobilized water molecules in the networks of DXs and RP-DX mixtures, the friction force among the DX molecules, and the hydrogen bonds formed between DXs and RP were responsible for the viscosity improvement of RP yogurts containing in-situ DXs. This study may guide the application of DXs in plant-protein food and prompt the exploitation of plant-protein resources.