To　improve　the　driving　safety　of　a　semitrailer　truck　on　curved　downhill　sections　of　a　freeway,　a　dynamics　model　integrated　with　vehicle,　road,　and　driver　models　was　established　using　TruckSim　software.　Based　on　the　Monte　Carlo　reliability　analysis　method,　the　performance　functions　of　skidding,　rollover,　jackknifing,　and　system　failures　were　developed.　A　freeway　with　a　design　speed　of　80　km•h-1　was　selected　as　the　research　section.　Through　extensive　vehicle　dynamics　simulation　tests,　a　numerical　analysis　that　assessed　the　effects　of　different　horizontal　curve　radii,　longitudinal　grades,　pavement　friction　coefficients,　speeds,　and　gross　vehicle　weights　on　the　lateral　stability　of　a　six-axle　semitrailer　truck　was　presented.　The　results　demonstrate　that　the　failure　probabilities　of　skidding　and　rollover　decrease　considerably　with　increasing　horizontal　curve　radii.　Under　the　condition　of　a　general　minimum　horizontal　curve　radius　of　400　m,　the　failure　probabilities　of　skidding　and　rollover　are　approximately　0,　and　the　probabilities　of　skidding　and　rollover　increase　linearly　with　an　increase　in　the　longitudinal　grade.　Moreover,　the　effect　of　speed　is　particularly　marked　on　the　running　security　system　of　a　semitrailer　truck.　When　the　mean　speed　increases　from　60　km•h-1　to　90　km•h-1,　the　probabilities　of　skidding　and　rollover　increase　by　634　and　336　times,　respectively.　The　gross　vehicle　weight　also　has　a　considerable　effect　on　the　rollover　of　a　semitrailer　truck.　On　low-adhesion　roads,　the　main　types　of　accidents　that　the　semitrailer　truck　experiences　are　skidding　and　jackknifing,　whereas　on　high-adhesion　roads,　the　main　accident　type　is　rollover.　This　indicates　that　the　combined　alignments　of　a　limited　minimum　radius　and　steep　downgrade　should　be　avoided　in　road　design.　Finally,　strict　limits　on　speed　and　gross　vehicle　weight　should　be　adopted　to　guarantee　the　safe　performance　of　vehicles.　©　2019,　Editorial　Department　of　China　Journal　of　Highway　and　Transport.　All　right　reserved.