For developing the X-ray Free Electron Lasers test facility (SXFEL) at Shanghai Institute of Applied Physics, Chinese Academy of Sciences (SINAP), ultra-short bunch is the crucial requirement for excellent lasing performance. It is a big challenge for deflecting cavity to measure the length of ultra-short bunch, and higher deflecting gradient is required for higher measurement resolution. X-band travelling wave deflecting structure has features of higher deflecting voltage and compact structure, which has good performance at ultra-short bunch length measurement. In this paper, a new X-band deflecting structure is designed to operate in HEM11-2,π,/3 mode. For suppressing the polarization of deflecting plane of the HEM11 mode, two symmetrical caves are added on the cavity wall to separate two polarized modes.