As the proportion of renewable energy power generation in the grid continues to increase, the problem of power system small-signal stability caused by grid-connected inverters has become increasingly prominent. To clarify the power margin of the grid-connected inverter system and improve the small-signal stability of the system, the impedance method is used to study the small-signal stable power limit of the inverter. From the boundary conditions of the generalized Nyquist stability criterion, the small-signal stable power limit of the grid-connected inverter is quantitatively calculated. It is pointed out that the power limit is less than the static stability power limit, and decreases with the decrease of the grid short-circuit ratio, and decreases with the increase of the phase-locked loop bandwidth. An improved phase-locked loop structure is proposed to increase the grid short-circuit ratio from the control point of view, thereby improving the small-signal stable power limit of the grid-connected inverter. Finally, the effectiveness of the improved phase-locked loop in improving the small-signal stable power limit is verified by impedance analysis and system simulation. This work is supported by the National Natural Science Foundation of China (No. 51707106).