The Loess Plateau of China is situated in a seismic zone, and its poor structural characteristics result in a particularly complex disaster mechanism. Improving the poor structural characteristics of loess is a crucial issue in seismic research for loess areas. In this study, Oxalic acid stands out for its superior environmental friendliness and biodegradability compared to traditional modifiers, which was used to dissolve and replace soluble salts in soil to improve the dynamic shear resistance of soil. The dynamic shear modulus and maximum dynamic shear modulus characteristics of OA-modified loess (OAML) under different influencing factors were studied by cyclic torsional shear test through a hollow shear apparatus. Scanning electron microscopy (SEM) experiments were conducted to investigate the microscopic characteristics of OAML. The results showed that the absolute increase in the dynamic shear modulus of OA-modified loess compared to the untreated loess was approximately 43.1 MPa, representing a relative increase of 33.60%. The three factors influencing the OAML dynamic shear modulus were OA concentration (c), water content (w), and confining pressure (63). ANOVA confirmed that the Hardin model parameters of OAML were significantly affected by c (strong positive), w (medium negative), and 63 (weak positive). Therefore, the OA modification index M was introduced, and an OAML modified dynamic constitutive model based on the Hardin-Drnevich model was established. The dynamic constitutive model applies to oxalic acid modified remolded Malan loess under dynamic triaxial loading within the test ranges of oxalic acid concentration, water content and confining pressure, and can accurately describe its dynamic stress-strain behavior. Scanning electron microscopy (SEM) images showed that the modification mechanism of OAML was to form more stable soil aggregates through the filling, skeleton support formation, and flocculation of oxalate crystals. These research results provide certain references and inspirations for the study of improvement of soil dynamic properties in the loess region.