<p>This study proposes an inserted steel plate shear wall (I-SPSW) to improve the constructability of modular cores and preliminarily evaluates its structural performance through finite element analysis (FEA) before experimental testing. The proposed system enhances constructability by using H-shaped columns and two C-shaped steel beams, with steel plates inserted between the beams and fastened by bolts. The FEA under cyclic loading was performed on a 1/3-scale, single-span, two-story structure, with variations in aspect ratio, steel plate thickness, and boundary frame–steel plate connection. The results indicate that increasing the aspect ratio slightly improves initial stiffness, strength, and ductility but also increases bending demands and stress concentrations in the boundary frame. Increasing the steel plate thickness enhances stiffness and strength but reduces ductility, introducing excessive deformation and a risk of buckling in the boundary frame. The boundary frame–steel plate connection configuration significantly influenced structural behavior. When the column and steel plate were not connected, stiffness decreased, but stress concentration in the column was mitigated. Partial connections between the steel plate and the boundary frame reduced maximum lateral strength. When the steel plate size was reduced and connected to only part of the beams, the structural response was similar to that of the boundary frame alone, indicating a negligible contribution from the steel plate. Overall, the analysis demonstrates that the I-SPSW exhibits both high structural performance and improved constructability. However, full-scale experiments are required to accurately evaluate the structural performance of the I-SPSW system.</p>