Abstract: To increase the power density and reduce the manufacturing costs, the inductive filter used by the grid-connected inverters (GCIs) becomes smaller and smaller, and its parallel with the inverters leads to the amplification of the equivalent grid impedance, resulting in a large ratio between the grid impedance and the filter inductance. This large ratio may cause the instability of the GCIs, while the causes of the instability are rather different from that in the traditional low short-circuit ratio (SCR) weak grid. By dividing the working conditions of the GCIs into the low SCR-low impedance ratio condition and the low SCR- large impedance ratio condition, the causes of the instability in these two conditions are compared and studied in this paper. It is found that the instability in the low SCR-low impedance ratio condition is mainly related to the negative impedance introduced by the PLL, while the instability under the low SCR-large impedance ratio condition is related to both the negative impedance introduced by the PLL and the negative impedance and capacitive impedance introduced by the grid voltage feedforward (GVF). In the low SCR-large impedance ratio condition, a small or a large GVF coefficient may both destabilize the system. To enhance the system stability under the low SCR-large impedance ratio condition, the optimal value of the GVF coefficient has to be used. Therefore, an optimization for the GVF coefficient is proposed based on the singular value of the system. Finally, simulation and experimental results verify the effectiveness of the research results and the proposed method.