A detector was developed for measuring ultra-wideband (UWB) transient electromagnetic field in the free-space .It is a small TEM horn antenna with gradually changed impedance. The major part of this paper is to describe the method of calibrating the detector’s sensitivity .Results show that in the range of 50Mhz~ 2.2Ghz, the detector’s sensitivity is with linear phase and a uniform magnitude within 20% . The error of calibrating is less than 12% .