Abstract: To address the aerodynamic force fluctuations caused by periodic vortex shedding in subcritical flow past a circular cylinder, a Negative Stiffness Cylindrical Shell (NSCS) is proposed and studied using a two-dimensional numerical model. The purpose is to explore a wake-control approach based on deformable negative stiffness units and to clarify its underlying mechanism for circular-cylinder flows. In the proposed configuration, arch-shaped unit cells with negative stiffness are distributed along the outer surface of the cylinder to form a reconfigurable boundary. Finite element analysis is first carried out in ABAQUS to verify that the unit cell possesses a negative stiffness interval, thereby confirming its ability to undergo snap through-like deformation. A two-dimensional numerical model for flow past the NSCS at Re = 20000 is then established, and the computational results are validated through comparison with published data. Furthermore, an User Defined Function (UDF)-based dynamic mesh method is employed to realize periodic indentation-recovery deformation of the unit-cell region, so that the effect of the deformable boundary can be incorporated into the flow simulation. On this basis, the aerodynamic response and vortex-shedding dynamics of the NSCS are systematically investigated at three deformation frequencies, namely 1 Hz, the dominant vortex-shedding frequency, 33 Hz, and its harmonic frequency, 66 Hz. In addition, the phase portrait of lift coefficient is introduced to characterize the periodicity, modulation, and lock-in/nonlinear interaction features of the lift response. The results show that, compared with the smooth circular cylinder, the static NSCS enhances the fluctuation amplitudes of both lift and drag and significantly reduces the Strouhal number ( S_t ), indicating that geometric modification alone does not weaken the wake unsteadiness. In contrast, the dynamic snap through boundary enables frequency-dependent active regulation of the wake response relative to the static NSCS. Specifically, the 1 Hz case mainly induces low-frequency envelope modulation and a certain suppression effect on the lift response; the 33 Hz case exhibits a more pronounced lock-in synchronization behavior when the excitation frequency matches the natural vortex-shedding frequency; and the 66 Hz case leads to stronger nonlinear interaction, as evidenced by divergent phase trajectories and an increased number of saddle points. These results indicate that the main advantage of the negative stiffness interface lies not in the static geometric modification itself, but in its ability to achieve programmable regulation of the wake dynamics of a circular cylinder through dynamic snap through behavior, thereby providing a theoretical basis for the array-based design of reconfigurable flow-control interfaces.