Improving antenna performance at low frequency is the focus of TEM horn antenna research. A new TEM horn antenna was designed by analyzing the impedance gradient and time-domain radiation characteristics of the antenna, to solve the reflection problem at the end of the antenna and improve the low-frequency performance of the antenna. Compared with the typical TEM horn antenna, the new TEM horn antenna designed in this paper reduces the pattern back lobe in the full frequency band, improves the antenna radiation spindle gain at low frequency, and expands the low frequency bandwidth. The antenna size is 48.5 cm×38.1 cm×35 cm. The experimental results show that the return loss at 160 MHz, 2.5 GHz is less than -10 dB and the pattern back lobe is less than -2 dB.