| The flexible energy converter (FEC) based on the traditional droop control method can realize seamless switching between islanding and grid-connection. However, due to the lack of inertia, the system is easy to generate frequency mutations when the output power command has a sudden change, which affects the stability of the system. In this paper, a low pass filter (LPF) is added to the FEC to make the frequency change smoothly, realizing the inertia effect of a synchronous generator equivalently. If the cutoff frequency of LPF is lower, then the simulated inertia effect will be bigger; nevertheless, the slower adjusting speed of the inverter will generate energy overshoot and make the adjusting time longer. Based on the inverter’s flexible parameter adjusting capability, an adaptive droop control is proposed, with which the time constant T of LPF can change adaptively according to inverter output situation. In this way, the FEC not only has the inertia that is needed to suppress the sudden change of output power demand, but also can reduce the overshoot of power and thus converge to a steady state more quickly. Meanwhile, this method is used in synchronization before grid-connection to keep the inertia and finish quickly. Finally, a transient energy analysis is performed for the proposed adaptive droop control when the output power command from the FEC has a sudden change, which verifies the feasibility and effectiveness of the proposed method theoretically; moreover, simulations and an experiment also verified the novel method .