With the rapid development of electric and electronic power system, grid-forming inverter as a new power source has been widely concerned. The single voltage loop grid-forming inverter has a unique advantage in new energy grid-connected because of its better small signal stability. Meanwhile, the transient response of grid-forming inverter is different from synchronous generator under large disturbance. Grid-forming inverter is prone to transient instability. For this problem, based on the large disturbance model of single voltage loop grid-forming inverter, the transient behavior of power control loop and voltage control loop under large disturbance is analyzed by the phase portrait, the influence of key controller parameters on the transient stability is given, and the transient response of power angle are described. It is revealed that the active power control loop, reactive power control loop and single voltage loop change the power angle overshoot, the output voltage amplitude, and the rate of change of the inverter, respectively. And they will affect the transient stability of the single voltage loop grid-forming inverter. Based on the above analysis, this paper presents a control parameter optimization method to reduce the active and reactive droop coefficients, increase the voltage integral coefficient and low-pass filter cutoff frequency, which can enhance the transient stability of single voltage loop grid-forming inverter. Finally, the accuracy and feasibility of the analysis are verified by the Matlab/Simulink.