This paper proposes a power controller design method for grid-forming inverters (GFMIs) in the islanded microgrid aiming at better transient power sharing performances. First, the studied system is modeled by a new small-signal model, where one of the GFMIs is modeled as a state observer to obtain the frequency and amplitude dynamics of point-of-common-coupling (PCC) bus voltage so that the frequency interaction between the PCC bus and the terminal bus of each GFMI is considered. Then, the generalized transient power sharing conditions for GFMIs with predominantly inductive line impedance and the design procedures of power controller are derived. Finally, a global power decoupling strategy is introduced to eliminate the influence of the resistance of line impedance so that the same design principle can be applied for GFMIs with resistive-inductive line impedance. The proposed small-signal model, power controller design method and power decoupling method are verified by both simulation and experimental results.