Abstract: In order to reduce the reflection from either the end of the monocone or the edge of ground plane and expand the time window of the mirror monocone TEM cell, the voltage reflection coefficients of the feed port under different resistively loaded conditions were calculated based on the 0.9 m monocone TEM cell, and the simulation results were verified by building a resistor array loaded monocone TEM cell experimental platform. The first low-reflection monocone microwave anechoic chamber in China was designed and built by optimizing the structure of the monocone TEM cell on the basis of improving the cone support structure and adjusting the distance between the ground plane and the absorbing material. The results show that loading resistor array between the end of the cone and the edge of ground plane can reduce the reflections at the end of the monocone TEM cell, breaks through the time window limit that is determined by the length of the cone, expands the application scope of the monocone TEM cell, and realizes full calibration of amplitude sensitivity and waveform fidelity of HEMP, LEMP and other transient electromagnetic pulse sensors.