Abstract: In order to detect the width and direction of cracks in metal structures, and improve the sensitivity and reliability of detection, a dual-polarization chipless RFID sensor based on frequency signals is designed. The sensor is a disc and dual-mode disc multi-microstrip resonator structure. The quad-band segment design is performed in the frequency range of 2.2-6.2 GHz. The structure optimization and performance simulation of the sensor are performed using HFSS software. The results show that the sensor generates two sets of four-position resonances when excited in the x and y polarization directions. The direction of the metal crack is judged according to the direction of the resonance frequency shift. Two polarization modes generate a total of eight bits of resonance shift information for determination, which can accurately identify cracks in the directions of 0°, 45°, 90°, and 135°. The crack width in the direction is fitted to the resonance frequency offset of the sensor, and the two have a good linear relationship, and the sensitivity of the sensor is greatly improved through polarization multiplexing. The resolution can reach sub-millimeter level, and the sensitivity in all directions is 40.71 MHz/0.1 mm, 11.28 MHz/0.1 mm, 26.04 MHz/0.1 mm, 15.19 MHz/0.1 mm respectively. The RFID sensor designed in this paper has the characteristics of passive, low cost and high sensitivity, and has potential for application in structural health monitoring.