Abstract: To improve the insufficient voltage and frequency support of renewable energy power systems composed solely of grid-following (GFL) inverters, the current approach is to introduce grid-forming (GFM) inverters into the system and construct a GFL-GFM hybrid system, in order to fully leverage the support capabilities of grid-forming equipment and maintain the stability of the renewable energy power system. However, due to the significant differences in control strategies and external characteristics between GFL and GFM inverters, as well as the mutual coupling phenomenon between the various inverters, the stability mechanism analysis of future hybrid systems will be more complex. However, existing research mainly focuses on the stability analysis of a single type of inverter, without delving into the stability conditions and support mechanisms of GFL-GFM hybrid systems. To address this issue, this article has derived the key conditions for the stability of GFL-GFM hybrid systems and revealed the supporting mechanism of the inverters in the hybrid system. First, the stability conditions for the hybrid system are derived. On this basis, the "hybrid system support quantity" is defined to characterize the stability of the hybrid system, and the "improved short-circuit ratio" is further derived to characterize the strength of the hybrid system. Then, the influence of inverter parameters on system stability is analyzed, and the critical capacity proportion of the inverter is quantitatively obtained. Finally, the correctness of the theoretical analysis is verified through experiments.