Molecular dynamics simulations of polyethylene/silver-nanoparticle composites are implemented to calculate the structures, electrical, thermal and mechanical properties, thereby investigating their relationships with the nanoparticle dimension and simulation temperature. The results show that polyethylene/silver-nanoparticle composites are of isotropic amorphous structure, and the dispersion of nanoparticles in composite can be enhanced at a relatively higher temperature. Multi-layers of atoms on nanoparticle surface change into amorphous configurations, and electrical polarization interface layers are formed between silver nanoparticles and polyethylene matrix. The interface region shrinks and expends respectively with nanoparticle dimension and temperature increasing. Compared with polyethylene system, the polyethylene/silver-nanoparticle composite presents explicitly high polarizability which increases with temperature and nanoparticle size rising simultaneously. The silver nanopaticle dimension directly influences the intensity and frequency of interfacial dipole moment, resulting in corresponding variations of peak position and intensity in infrared spectrum. The polyethylene/silver-nanoparticle composite also shows higher isometric heat capacity and negative thermal pressure coefficient with better temperature stability, which decreases explicitly with temperature and nanoparticle size increasing respectively, than polyethylene system. The mechanical property of polyethylene/silver-nanoparticle composite shows isotropic elastic constant tensor with considerably higher Young modulus and Poisson ratio than the polyethylene system, both of which decrease with temperature and nanoparticle dimension increasing, which indicates the improvement on mechanical property with Ag nanoparticle filler.