Abstract: To investigate the low-frequency oscillation of renewable-energy grid-following inverters under very weak grid where the short circuit ratio is nearly 1.0, a simplified small-signal model that describes low-frequency dynamic of the inverter is established in this paper. Based on the model, the main factor that causes the oscillation is analyzed. Researching results suggest that under very weak grid, the AC voltage control (AVC) and active power control are tightly coupled to each other, which causes serious interference between them and results in the low-frequency oscillation. Whether the oscillation happens or not mainly depends on dynamic of AVC. This paper proposes a stabilizing method based on transient reactive-power over-compensation, which expedites AVC through feeding forward. The method effectively suppresses the low-frequency oscillation under very weak grid, which improves the stability apparently. Compared with previous stabilizing methods, the proposed one has three advantages, i.e., easy parameter tuning, low calculating power requirement and never weakening the power control dynamic performance. At last, the analyzing results and stabilizing method are verified by both simulation and experiment.