A Transverse Electro Magnetic wave(TEM) horn is designed with floor boarding structure and plexiglass antenna cap. The key physical dimension is preliminarily determined by the analysis on antenna radiation characteristic and feed-in structure. The objective antenna model is established by CST. Relevant structural parameters of TEM horn are optimized by simulation. It is realized that reflection of short pulse is small at antenna feed-in, and the TEM horn has preferable time-domain and frequency-domain characteristics. The test results indicate that, the input impedance of TEM horn is 56 Ω, the effective height is 0.024 m, the transfer function keeps stable within 3 GHz, and the TEM horn is fit for ultra-wideband reception antenna.