<form id="form1" name="form1" method="post">

  <label for="daan"><p>请填入您的答案：</p></label>

  <p>

    <input type="text" name="daan" id="daan"/>

  </p>

  <input type="submit" name="submit" id="submit" value="提交"  disabled="disabled" />

</form>

<form id="form1" name="form1" method="post">

    <label for="daan"><p>请填入您的答案：</p></label>

    <p>

        <input type="text" name="daan" id="daan"/>

    </p>

    <input type="submit" name="submit" id="submit" value="提交"  disabled="disabled" />

</form>

<script type="text/javascript">

document.getElementById("daan").onblur=function(){

    document.getElementById("submit").disabled=this.value=="";

}

</script>