Abstract: Under the goal of "carbon peak and carbon neutrality", renewables such as wind power and photovoltaic, using power electronic converters as grid-connected interfaces, are developing rapidly. Grid-following (GFL) and grid-forming (GFM), as the two mainstream control methods of power electronic converters, have attracted widespread attention for their stability when connected to the grid. Existing studies have compared the differences in impedance characteristics and oscillation risk between GFM and GFL based on the impedance model and Nyquist stability criterion. However, since GFM and GFL have multiple control loops, the difference of their characteristics in different frequency bands and their dominant factors are still unclear, especially the role and function of the synchronization loop. In addition, the mechanism of GFM connected to the grid in improving the stability of GFL remains to be analyzed. In this paper, the impedance characteristics of GFL and GFM in different frequency bands are compared and analyzed, the role and function of synchronization loop in the impedance characteristics are clarified, and the dominant factors that determine the difference between the impedance characteristics of GFL and GFM are revealed. Then, the three-port model of GFL and GFM connected to the grid is also established, and the mechanism of GFM connected to the grid in improving the stability of GFL is analyzed. Based on the control hardware-in-the-loop simulation platform, the GFL and GFM grid-connection experiments are carried out to verify the correctness of the analysis in this paper.