A Fano resonant structure based on the coupling of the photonic crystal nanobeam with the photonic crystal slot nanobeam cavity is proposed to improve the sensing performance of the photonic crystal resonant cavity. A slot structure is designed in the nanobeam cavity with conical holes to obtain a highly localized light field that produces strong light-matter interaction and ultimately to improve the sensitivity. The model is simulated by the three-dimensional finite-difference time-domain method, and it is found that the sensitivity of the refractive index sensor with the proposed structure reaches up to 897 nm/RIU, and the size of the proposed structure is only 21.4 μm ×1.55 μm. In light of the coupled-mode theory, the mechanism of Fano resonance is discussed and the variation of the transmission characteristics of the proposed structure with the structural parameters is analyzed qualitatively.