Bound rubber plays a key role in the mechanical reinforcement of elastomer nanocomposites. In the present work, we reveal the formation mechanism of bound rubber in elastomer nanocomposites, using the coarse-grained molecular dynamics simulations. For the polymer-nanoparticle system, the "chain bridge" connected with neighboring nanoparticles forms, once the gap between two neighboring nanoparticles is less than the polymer size. The polymer-nanoparticle-solvent systems, mimicking the oil-swollen rubber in the experiment, are simulated with three models. From the analysis of the potential energy, the static structure and dynamic diffusing processes, all the models indicate that the increase of the volume fraction of the nanoparticles and the polymer-nanoparticle interaction strength could promote the formation of the bound rubber. The existence of solvent disrupts the bound rubber, and eventually deteriorates the mechanical properties. These simulations could provide some theoretical guidance for a better understanding of the formation mechanism of the bound rubber, which is helpful for designing the elastomer materials with excellent mechanical properties.