Abstract: In order to eliminate the error caused by the simplification of the riser below mudline to fixed constraint in the mechanical characteristic analysis of the riser in the offshore exploratory well, the D’Alembert principle and the micro-element balance equation were used, in combination with the Morison equation and API guidelines. The riser was divided into an above-water segment, an underwater segment, and a seabed soil-embedded segment. A more accurate theoretical mechanical model was constructed by considering wind/wave current load and pipe–soil interaction. Through linearization and Fourier series expansion, the equivalent transformation of nonlinear terms was realized. Finally, the partial differential equations of the theoretical mechanical model were transformed into first-order ordinary differential equations for iterative solutions. The model verification shows that the calculated results are highly consistent with the finite element simulation, and the maximum lateral displacement error is only 0.84 m (when the wave height is 13.8 m), which confirms the reliability of the model. The case study shows that the mechanical response of the riser shows obvious spatial characteristics: the maximum lateral displacement occurs at 30 m underwater; the maximum rotation angle is located at the top of the above-water segment, and the maximum von Mises stress is concentrated in the soil-embedded segment below the mudline (about 260 MPa). Parameter sensitivity analysis reveals that the lateral displacement of the riser increases by 146% when the wave height increases by 6.9 m (from 13.8 m to 20.7 m). When the flow velocity increases from 1.58 m/s to 2.32 m/s, the stress level increases by 23%. The stress decreases by 15% when the thickness of the riser increases by 6 mm. Raising the top tension by 200 kN reduces the lateral displacement by 28%. The change in the internal friction angle of sand has a weak effect on the mechanical response of the riser (the lateral displacement only increases by 5% when the internal friction angle decreases by 10°). The results indicate that the iterative calculation approach for determining the soil influence depth replaces the conventional fixed constraint assumption of six times the casing diameter. By this way, it improves the computational accuracy of theoretical mechanical model, providing a more rational framework for the analysis of riser mechanical response.