Abstract: The paper analyzes the response equation of TEM horn to Gaussian pulse based on the theory of transmission line. The dependence of TEM horn’s response on its open angle is numerically studied under the excitation of Gaussian pulse and its differential, respectively, according to the tail fluctuation and negative peak undershoot in response. The experiments are conducted based on the time-domain test system with cone TEM cell. The results show that, for differential Gaussian pulse of 1 ns bottom-width, the negative peak undershoot increases from 1.0% to 3.6%, as the open angle varies from 8° to 20°. The experimental results are consistent with theoretical analysis and numerical simulation.