Under severe grid faults, the grid-connected inverter system faces the problem of synchronization instability. If the fault is not monitored and removed in time, the grid may even collapse. Therefore, the research on transient instability mechanism and stability improvement method of grid-connected inverter system is of great significance. To the end, this paper analyzes the effects of PLL (phase-locked loop) parameters and injection current mode on the transient stability of the system by using equal area method, and shows that freezing the integral controller of PLL and injecting reactive current are both beneficial to improve the grid-connected inverter transient stability. Based on this conclusion, this paper proposes a transient stability improvement scheme for grid-connected inverters based on instability prediction and adaptive parameters adjustment. By monitoring the d-axis voltage and Δω_pll obtained in PLL, system instability can be predicted in time and parameters can be adjusted quickly in order to improve the transient stability of the system. Finally, the effectiveness of the improvement scheme is verified by simulation.