An optical sensing structure was designed to detect the single Nanoparticle. The structure consists of a dual-ring, a nanopore in the coupling area and a straight waveguide, and the Fano effect is introduced. The proposed structure could further amplify the change of coupling effect due to the presence of nanoparticle. Nanoparticles flowing through a nanopore between microrings could change both the coupling coefficient and the optical intensity at the output. An ultrahigh precision method for coupling coefficient sensing was proposed based on a dual-microring resonator structure. The counting and sizing of nanoparticle could be achieved by detecting the change of optical intensity and coupling coefficient. The theoretical results show that the sensitivity of the dual-ring design is calculated to be around two orders of magnitude greater than a single-ring design under 1dB/cm loss condition. The propose structure can improve the sensitivity effectively with reducing the waveguide loss.