<p>Existing Modular Multilevel Converter (MMC) port impedance models and system stability studies primarily utilize conventional Phase-Locked Loop (PLL) or standard Dual Second-Order Generalized Integrator PLL (DSOGI-PLL) structures, which exhibit suboptimal phase-locking performance. Further research is required on developing models and stability analysis using faster, more accurate PLLs. This paper proposes an MMC port impedance calculation method based on the multi-harmonic linearization approach with the improved DSOGI-PLL. The impedance model is established and validated using sweep frequency testing. The effects of control parameters on port impedance are analyzed, and the MMC phase-locking performance under different PLL strategies is compared. The Nyquist stability criterion is used to assess grid-connected MMC system stability, revealing parameter influence trends. The results demonstrate that the PLL and current loop control have been demonstrated to affect port impedance significantly, and reduced current loop gain and lower grid strength have been shown to lead to instability. However, the damping coefficient added to the improved DSOGI-PLL does not cause system instability. Moreover, the impedance model constructed based on the improved DSOGI-PLL provides more accurate analytical results. The validity of these findings is further substantiated by the corroboration from MATLAB/Simulink simulations, which validates the accuracy of the theoretical analysis.</p>