The bimetallic nanoparticle can effectively integrate the physical and chemical properties of two metals and simultaneously exhibits the unique natures of each metal. It also serves as a good candidate for improving light scattering, photothermal conversion, plasmon resonance decay, and photon excitation. Investigating the optical properties of an individual nanoparticle can avoid the interaction between nanoparticles during experimental research, which allows us to more effectively analyze the interaction between the incident light and nanoparticles. In this work, the finite-difference time-domain method is used to study the spectral absorption characteristics of the plasmon bimetallic core-shell nanoparticles by calculating the spectroscopic properties, and also the distributions of the magnetic field, electric field, and absorption power during energy transmission and decaying. The results show that the resonance wavelength is red-shifted if the core diameter is increased. In addition, the absorption rate of Ag@Pt bimetallic nanoparticles is higher than that of pure Ag@Ag nanoparticles when the core diameter is bigger than 100 nm. This is because the strong shielding effect between the shell metal material and the core metal material leads the incident light to interact only with the outer atoms, resulting in resonance. Meanwhile, the plasmon of the Ag core decays faster than that of the Pt shell and more energies are transferred to the Pt shell. As a result, the surface of the Pt shell shows more concentrated magnetic and electric fields associated with an enlarged absorbing power. Moreover, the energy in the Ag core tends to transfer to the nearby Pt shell, which is characterized by the energy absorption in the Pt shell close to the Ag core, and is more concentrated. This paper provides theoretical guidance for designing plasmon bimetallic core-shell nanoparticles, thereby satisfying the demands for special spectral responses.