When the load of an islanded microgrid changes, it can result in a frequency deviation from the nominal value, necessitating secondary control to restore the frequency to the nominal value. Traditional decentralized secondary frequency control relies on proportional-integral feedback for zero frequency deviation control, but it undermines active power sharing. In order to achieve active power sharing in traditional decentralized frequency secondary control, taking droop control function as a starting point, this study deduces and summarizes the conditions for active power sharing and zero frequency deviation control, as well as the basis for the adjustability of the droop control function and the rules for adjusting the droop control function in decentralized frequency control for microgrids. Then leveraging the derived mechanisms, a decentralized frequency control method is proposed that achieves both zero frequency deviation control and active power sharing by adjusting the droop control function. The proposed method resolves the active power sharing limitations encountered by traditional decentralized frequency secondary control methods. It requires no communication, is straightforward to implement, and is applicable to distributed generation control with different nominal active power. Simulation results are provided to validate the proposed method.