Expansive soils exhibit notable volume changes when moisture levels fluctuate. Understanding the dynamic behavior of these soils is essential for solving geotechnical engineering issues, as it helps in predicting their response to dynamic loading. However, earlier research have focused on non-expansive soils, leaving a gap in knowledge regarding the dynamic properties of fibre-reinforced expansive soils. Soil exhibits anisotropic, nonlinear, and time-dependent behavior when subjected to significant ground forces. Fibre-reinforced soils, in particular, display nonlinear elastic responses under dynamic loading. This study aims to analyze the nonlinear elastic properties of fibre-reinforced expansive soil under cyclic loading, with the goal of estimating the shear modulus and dynamic ultimate stress using theoretical analytical models. The nonlinear behavior is characterized by validating ten constitutive coefficients through linear regression analysis, accounting for various influencing factors. These coefficients represent the influence of fibre content, number of cycles, water content, and confining pressure on the dynamic shear modulus, and dynamic ultimate stress, enabling prediction of soil response under dynamic conditions. By integrating cyclic triaxial test data from both unreinforced and fibre-reinforced soil samples, theoretical models were utilized to calculate the dynamic shear modulus. The dynamic shear modulus can be expressed as a function of variables such as fibre content, number of loading cycles, initial moisture content, and confining pressure. Results present that each of these parameters independently affects the behavior of the reinforced soil. Regression analysis provided key parameters for the shear modulus model, including a0, a1, a2, a3, and a4 for the ultimate dynamic stress model, including b0, b1, b2, b3, and b4. The inclusion of fibres in the soil notably improves both the secant shear modulus and the dynamic ultimate stress, as evidenced by the fibre impact factors, which underscore their significant influence on the dynamic shear modulus and ultimate dynamic stress. Based on the findings, the improvement in dynamic properties due to fibre reinforcement can be effectively utilized in construction and soil stabilization, particularly in regions with expansive soils subjected to dynamic loads.