var startY = 0,endY = 0,offset = 0;

 document.addEventListener("touchstart", function (e) {
      var touch = e.touches[0];
      startY = Number(touch.pageY);
 })

 document.addEventListener(‘touchmove‘, function(e) {
    var obj = document.documentElement,
        touch = e.touches[0];
        if (obj.scrollTop == 0) {
           
           if (touch.clientY > startY) {
　　　　　　　　　　// 下拉到顶
                 e.preventDefault();
              } else {
                 e.stopPropagation();
              }
        }
 },{passive:false});