The oat dough lacks viscoelasticity and hinders its utilization in noodles. In this study, oat flour was modified by ethanol–water treatment, the changes in starch structure and properties during modification, and the influence of modified oat flour on the processing qualities of noodles containing 50% oat flour was investigated. The granular cold–water swelling starch (GCWS) was prepared during the modification, and the A–type starch crystal transformed to the V–type. No endothermic peaks were observed in the DSC thermograms. Modified oat flour has an advantage over oat flour in that it has higher cold water viscosity. As the modified oat flour content rose from 0% to 10%, the tensile strength of dough sheet increased from 345.57 g to 367.19 g, the transverse relaxation time (T₂) of water molecules increased. Water absorption of cooked oat noodles rose from 125.18% to 129.32%, with a reduction in cooking loss from 10.12% to 8.97%. The shear force, which indicates the hardness of cooked oat noodles, showed a decline of from 210.83 g to 190.02 g. The mentioned results indicated the ethanol–water treatment prepared GCWS, and that improves the processing quality of dough sheets via the increase of its tensile strength.