A plasma refractive index nanosensor based on Fano resonance is designed, which is composed of the metal-insulator-metal waveguide with a stub coupled with a split-square resonator. Analysis of transmission characteristics of the sensor's structure is performed using the finite element method, and the influences of the structural parameters on the sensing characteristics of the sensor are studied. The calculated results show that the structure can excite the Fano resonance, and the resonance peak wavelength and line type can be adjusted by changing the key parameters. By adjusting the structural parameters, the sensitivity of the structure is up to 1125.7 nm/RIU and the figure of merit is 30.01. The results indicate that the proposed structure can be applied to optical integrated circuits, especially in nanometer biosensors.