The grid-forming-based renewable energy power generation units can provide inertial support to isolated power grids, but they face transient stability issues when faults occur. To address this issue, the impact mechanism of the reactive power outer loop of the grid-forming-based renewable energy power generation units on the transient stability of isolated power grids is studied, and corresponding stability improvement strategies are proposed. Firstly, the Lyapunov direct method is employed to explore the stability boundaries of the output-end voltage of the grid-forming-based renewable energy power generation units. Secondly, a comprehensive control strategy with an adaptive droop coefficient based on the stability boundaries of the output-end voltage is proposed. This strategy not only maintains system power angle stability but also limits fault currents. It enhances the transient stability of the system while ensuring the safety of inverters. Finally, electromagnetic transient simulations are conducted by using MATLAB/Simulink, which validate the correctness of the theoretical analysis and the effectiveness of the proposed transient stability improvement strategy.