To investigate the effects of polysaccharides from Tremella fuciformis with different molecular weights on the structure and physicochemical properties of oat starch, acidic extracts (ACE, approximately 37.4kDa) and alkali extracts (ALE, approximately 3570kDa) were separately mixed with gelatinized oat starch (GOS) to prepare eight composite products (GOS-5%ACE, GOS-10%ACE, GOS-15%ACE, GOS-20%ACE, GOS-5%ALE, GOS-10%ALE, GOS-15%ALE, and GOS-20%ALE). Using pure GOS as a control, the short-range ordered structure, micromorphology, thermodynamic properties, and viscoelasticity of GOS and the eight composites were compared. The results of amylose leaching measured by iodine colorimetry showed that compared to GOS, the amylose leaching of GOS-5%ACE and GOS-10%ACE significantly decreased, GOS-15%ACE and GOS-20%ACE showed no significant difference, while the results of the four composites in the GOS-ALE group significantly increased. Infrared spectroscopy analysis revealed that among the eight composites, GOS-10%ACE exhibited the strongest intermolecular hydrogen bonding. Scanning electron microscopy results indicated that both ALE and ACE disrupted the formation of the oat starch gel network structure during gelatinization to some extent. Thermal stability analysis showed that compared to GOS, the four samples in the GOS-ALE group had a slower water loss rate, while the four samples in the GOS-ACE group had a slower organic matter loss rate. Rheological test results demonstrated that the storage modulus of all eight composites was lower than that of GOS, and the higher the ACE addition, the lower the storage modulus. This study indicated that adding 5% to 20% of the two Tremella polysaccharides improved the thermal stability of oat starch and reduced its viscoelasticity, with ACE having a greater impact on the gel strength of oat starch. The findings provided theoretical support for the development of high dietary fiber oat starch products. 〖