The　vehicle　dynamic　load　can　greatly　affect　the　service　life　of　pavement.　To　study　the　vehicle　dynamic　load　accurately　and　conveniently　under　different　pavement　characteristics　and　driving　conditions,　a　multibody　dynamics　method　was　employed　to　establish　a　three-dimensional　(3D)　vehicle　dynamics　simulation　model　of　a　six-axle　heavy　vehicle　(the　most　typical　heavy　vehicle　on　an　expressway).　Experimental　tests　of　vehicle　component　parameters　and　axle　spindle　loads　were　performed　to　provide　simulation　parameters　and　to　validate　this　vehicle　dynamics　model,　respectively.　The　validation　results　showed　that　the　accuracy　and　robustness　of　the　dynamics　model　were　satisfactory.　Based　on　the　validated　model,　the　effects　of　International　Roughness　Index　(IRI)　and　vehicle　speed　on　the　dynamic　load　coefficient　(DLC)　were　explored.　The　DLC　increased　by　about　80%　when　the　speed　or　IRI　changed　from　30　km/h　or　1.184　m/km　to　70　km/h　or　3.551　m/km,　respectively.　To　better　evaluate　the　effect　of　dynamic　load,　the　relationship　between　the　DLC　of　the　vehicle　and　the　vehicle　speed　and　IRI　is　proposed　according　to　simulation　results.　This　relation　equation　can　quickly　and　accurately　study　the　DLC　of　this　heavy　vehicle　under　different　IRI　pavements　and　driving　speeds.　Because　the　vehicle　dynamics　model　is　convenient　for　studying　the　dynamic　load　under　different　conditions,　this　method　is　recommended　in　vehicle-pavement　coupled　analysis.