The excellent catalytic activity of Platinum (Pt) nanoparticles have been widely used in energy and energy storage. The investigation demonstrates that the catalytic capacity of Pt depends on the number and type of active sites on the surface, however, the regulation mechanism of its surface activity was not fully understood. In this paper, molecular dynamics method was used to study the microstructure and phase transformation of Pt nanoparticles based on LAMMPS software. The results show that the proportion of disordered atoms of Pt nanoparticles decreases with the increase of particle radius, and the melting temperature of Pt nanoparticles decreases with the decrease of particle radius. In addition, the particles can be further divided into two parts: the surface shell and the inner core. Like the bulk material, the coordination number of the inner core is also 12. The thickness of the shell was about 0.27 nm with the thickness close to 2 layers of atoms and the coordination number decreases with the increase of the core distance. This unique core-shell structure resulting that the potential energy surface atoms was approximately higher than the core. In this study, we found that the Pt shell atoms with particle radius of 3nm can melt at 1300K, whereas the inner atoms can not melt. Therefore, the structure characteristics of Pt catalyst can be regulated by the phase transformation law, which provides a theoretical basis for the regulation of surface activity.