var start="116.399,39.910"
for(var i=0;i<1000;i++){
start+=(';'+(116.399+Math.random()*i/2000)+','+(39.910+Math.random()*i/2000))
}
var polyline = new BMap.Polyline("116.399,39.910;116.405,39.920;116.425,39.900", {strokeColor:"blue", strokeWeight:6, strokeOpacity:0.5});
map.addOverlay(polyline);
setTimeout(function(){polyline.setPath(start)},2000)