Abstract: In this paper, a polarization conversion unit based on an asymmetric split ring structure is proposed and designed. Based on this design, an ultra-wideband RCS reduction electromagnetic metasurface is realized. The asymmetric split ring structure polarization conversion unit can flexibly control the polarization reflection phase. Two polarization conversion units are designed to achieve the reflection phase difference of nearly 180° in the ultra-wideband range. The unit is optimally coded on the plane to make the incident electromagnetic wave produce a diffuse reflection effect to achieve ultra-wideband backscatter RCS reduction. Finally, an electromagnetic metasurface of 192 mm×192 mm size is designed and fabricated. The simulation and measured results show that the surface achieves RCS reduction of more than 10 dB in the wide frequency band of 11.2-24.1 GHz, and the maximum reduction is up to 35 dB.