Resonance in subwavelength metal structures is very important in the field of nanosensors. This paper studies the resonance characteristics and sensing applications of nanocavities in metal structures. The related inclined cavity, V-shaped cavity and X-shaped cavity are constructed. The V-shaped cavity is composed of two inclined cavities and the X-shaped cavity is composed of two V-shaped cavities. The finite difference time domain method is used to simulate thecoupling effect of the three cavities and the waveguide, and the coupled mode theory is applied to verify the transmission spectrum. The standing wave theory is used to analyze the physical mechanism of three kinds of intracavity resonance. It is found that there is one resonance circuit in the inclined cavity and the V-shaped cavity, and there are two resonance paths in the X-shaped cavity. Compared with the inclined cavity, there is a connection mode in the V-shaped cavity; there are both symmetrical and antisymmetric connection modes in the X-shaped cavity. When the symmetry of the X-shaped cavity structure is broken, its resonance mode splits. Considering the differential sensing characteristics of the three resonant cavity structures, the average measurement errors of the gas refractive index sensor composed of the inclined cavity, the V-shaped cavity and the X-shaped cavity are respectively 0.003 91, 0.001 85 and 0.000 37, and the X-shaped cavity sensing is the most accurate. The multiple resonance circuits in the X-shaped cavity form multiple resonance modes, which effectively eliminates the interference of the changes of multiple environmental factors on sensing, thereby obtaining higher sensing measurement accuracy, which provides valuable theoretical reference for the application of differential sensors.