Based on the transmission characteristics of surface plasma sub-wavelength structure, a metal-dielectric-metal waveguide coupled ring cavity structure with metallic double-slit is proposed. The Fano resonance is formed by the coupled destructive interference between wide wave with continuous state generated by a metallic double-slit waveguide resonator and narrow wave with isolated state generated by a ring cavity. According to the coupled wave theory, the transmission and phase characteristics of Fano resonance formed by this structure are analyzed. The finite element analysis method is used to simulate the structure, and the effects of the structural parameters on the slow light effect are analyzed quantitatively to optimize the structural parameters. The results show that the optimized group index can reach 205. This structure can provide an effective theoretical reference for the design of integrated plasma slow light devices.