Facing on the contradiction between the coding capacity and the tag size in the design of chipless tags, a new chipless tag structure is proposed. The tag is composed of a substrate-integrated waveguide and a complementary split ring on the surface patch. The complementary split ring contains nested inner ring and outer ring. The resonance frequency of the tag can be tuned by adjusting the opening angle of the inner and outer ring of the complementary split ring, in which the outer ring is responsible for a wide range of frequency coarse tuning, and the inner ring is used for a small range of frequency tuning. The tag operates in the frequency range of 4 GHz to 6 GHz, the size is 25 mm×15 mm, and the coding density is as high as 4.86 bit/cm2. The radar cross section curve of the tag is obtained through simulation, and the consistency with the theoretical analysis is verified. Compared with the traditional chipless tag, the structure can improve the coding capacity without increasing the size of the tag, while the substrate-integrated waveguide provides high selectivity for the tag, which keeps the tag with high spectral resolution.