Based on the double Rabi splitting experiment in the methylene blue (MB) - silver nanocavity, a structural model of MB molecular clusters and dual metal nanoparticles was established. Using the density matrix theory framework and dipole approximation, the coupling dynamics of the hybrid state formed by MB molecular clusters and dual metal nanoparticles were calculated. The multi-mode coupling effect under the interaction between multi-exciton states and plasmons was studied, and qualitatively consistent results with the experiment were obtained. By short pulse excitation, the coupling states of exciton states and plasmons are studied in a larger excitation frequency domain. The paper explores the effects of exciton decoherence rate and intermolecular distance on the coupling process, as well as the phenomenon that the coupling strength between molecules and plasmons increases with the shortening of exciton decoherence time. Due to the coupling interaction between delocalized excitons and plasmons within the cluster, more hybrid energy levels can be generated in the composite system, resulting in corresponding changes in the optical response peak. By studying the mechanism of multi-mode coupling between molecular clusters and metal nanostructures, theoretical ideas have been provided for designing efficient light harvesting and conversion materials.