var divToMove = document.getElementById('divToMove');
var targetDiv = document.getElementById('targetDiv');

divToMove.style.position = 'relative';

var interval = setInterval(function() {
  var divRect = divToMove.getBoundingClientRect();
  var targetRect = targetDiv.getBoundingClientRect();

  if (
    divRect.top >= targetRect.top &&
    divRect.bottom <= targetRect.bottom &&
    divRect.left >= targetRect.left &&
    divRect.right <= targetRect.right
  ) {
    // 移动的DIV完全在目标DIV中
    clearInterval(interval);
    console.log('移动的DIV完全在目标DIV中');
  }
}, 100);