A surface plasmon polariton (SPP) waveguide structure composed of a straight waveguide with a metal baffle and square cavity is proposed. The magnetic field distribution, Fano resonance, and sensing characteristics are analyzed using the finite element method. The simulation results show that the SPP waveguide structure forms two Fano peaks, and the position of the Fano peaks can be adjusted using structural parameters. In addition, the SPP waveguide structure can be used for refractive index sensing. The maximum refractive index sensitivity and the figure of merit is 2220 nm/RIU (refractive index unit) and 5542, respectively. In glucose concentration detection, the sensitivity reaches 0.264 nm/(g·L^-1). The designed SPP waveguide may have potential applications in micro-nano-sensing fields.