The performance of In?Ga???N/GaN multiple quantum well (MQW) solar cells is critically influenced by structural parameters, primarily the quantum well period and the indium (In) composition. A numerical model is established using the Silvaco TCAD Atlas platform to systematically quantify the influence of these parameters for device optimization. It is shown that increasing the quantum well period significantly enhances the short-circuit current density (from 0.48 to 1.39 mA/cm2) and the power conversion efficiency (from 1.64% to 4.85%), although a clear saturation trend is observed. While a higher In composition broadens the spectral response by red-shifting the absorption edge, it also reduces the open-circuit voltage due to enhanced polarization. Furthermore, a dual-In-composition structure is demonstrated to improve both spectral broadening and carrier collection, raising the efficiency to 5.86%, which exceeds the 4.85% of the single-composition design. This work clarifies the regulatory mechanisms of these parameters and provides clear guidelines for designing efficient InGaN solar cells for full-spectrum utilization.