Localized surface plasmon resonance (LSPR) spectrum behaviors of PMMA/Ag core-shell nanoparticles are extensively and systematically simulated by a calculation method based on Mie theory. Through the simulation calculation, the dependence of the LSPR behaviors of the core-shell nanoparticle on nanoparticle size, ratio of the radius of PMMA core to the thickness of Ag shell, and some other factors is studied. Furthermore, LSPR behaviors of Ag nanoparticles are simulated as well for comparison. The simulation results show that the nanosized PMMA dielectric core can weaken the retardation effects in the process of free electron resonance in the Ag nanoshell. Thus, compared with the LSPR of Ag nanoparticles, the position of the LSPR peak of the PMMA/Ag core-shell nanoparticles has an even more obvious red shift and the width of the LSPR becomes even narrower. Such simulation results indicate that the core-shell nanoparticles would have more advantages over Ag nanoparticles for potential applications of future chemical and biological nanosensors.