您必须在JavaScript字符串文字中转义换行符:

You have to escape new-lines in JavaScript string-literals:

function asdf() {
    document.getElementById("qwerty").innerHTML="A<br>\
      B<br>\
      C<br>\
      D<br>";
}

尽管您可以(可能更容易)在字符串本身中插入换行符:

Though you could, potentially more-easily, simply insert newlines in the string itself:

function asdf() {
    document.getElementById("qwerty").innerHTML = "A<br>\nB<br>\nC<br>\nD<br>";
}