To investigate the influence of excavation width on the embedment stability of retaining structures, the slip surface is assumed to be a combination of a straight line and a logarithmic spiral. According to whether the logarithmic spiral slip surface can fully develop within the excavation width, excavations are quantitatively classified into ordinary excavations and narrow excavations. The energy method is employed to derive the moments of active and passive earth pressures about the lowest support point of the retaining structure for both ordinary and narrow excavations, respectively. Based on these derivations, a calculation formula for the embedment stability factor of safety that accounts for excavation width is established, followed by comparative verification and parametric analyses. The results indicate that the critical width is influenced by both the layout of the retaining structure and the soil parameters. The proposed method, which incorporates the excavation width, yields results comparable to those of current code specifications for ordinary excavations; however, for narrow excavations, significant discrepancies are observed, and the difference becomes more pronounced as the excavation width decreases. Furthermore, the factor of safety obtained by the new method decreases with increasing distance from the lowest support point to the excavation bottom, while it increases with increasing soil cohesion, internal friction angle, and friction angle between the soil and the retaining structure.