Weather　conditions　affect　driving　safety.　Study　on　the　characteristics　of　the　driver　workload　in　bad　weather　conditions,　which　can　improve　the　driving　safety.　A　random　sample　of　3　drivers　was　tested　in　a　simulator　cabin　that　simulated　driving　on　a　freeway　under　four　scenery　of　weather　conditions　which　are　clear　day,　moderate　rain,　moderate　rain-fog　(Visibility　100　m)　and　moderate　rain-fog　(Visibility　50　m).　Index　of　heart　rate　variability　(HRV)　is　adapted　to　measure　driver　workload.　We　found　the　driver　workload　varies　in　different　weather　conditions　and　it　is　higher　with　the　worse　of　the　bad　weather　condition.　Visibility　of　the　driving　environment　has　the　most　influence　on　the　driver　workload.