Abstract: In view of the fact that almost all the control parameters of the excitation system under excitation limiter (UEL) are fixed by default and conservative, and the lack of quantitative evaluation means for its stability, a practical method for evaluating the stability margin of generator unit during UEL action based on field test is proposed. The frequency domain characteristics of the closed-loop control system during UEL action are obtained by frequency scanning, and the Nyquist curve of the open-loop control system is obtained by combining the excitation regulation system model of the generator unit, so as to evaluate the stability margin of the system during UEL action by the relation between the curve through the real axis point and the position (1, j0), and to guide the setting and optimization of UEL control parameters. In the laboratory, the closed loop test is carried out on the real excitation regulator device which adopts the competitive type and the superimposed type UEL connected to the voltage control main loop respectively, and the feasibility and effectiveness of the test scheme and evaluation method are verified, which enriches the means of on-site UEL stability test.