<input name="ckID[]" type="checkbox" id="<?='ckID'.$row['MIS_id']?>" value="<?=$row['MIS_id']?>" onclick="showInfor(this.value)" />
<div id="txtHint">
<input name="txtid" type="text" id="txtid" />
</div>

function showInfor(str) { 
    //document.getElementById("txtid").value=str;
    //var str = document.geElementById("ckID").value;
    //document.write(str);
    var xmlhttp;
    if (id.length==0)
      {
      document.getElementById("txtid").value="";
      return;
      }
    if (window.XMLHttpRequest)
      {// code for IE7+, Firefox, Chrome, Opera, Safari
      xmlhttp=new XMLHttpRequest();
      }
    else
      {// code for IE6, IE5
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
      }
    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
        //document.getElementById("show").innerHTML=xmlhttp.responseText;
        document.getElementById("txtid").value=xmlhttp.responseText;
        }
      }
    xmlhttp.open("GET","showSettle.php?q="+str,true);    
    xmlhttp.send();
}

$db = mysql_connect("localhost","root","1234");
mysql_select_db("it database",$db);

$q=$_GET['q']; // connecting to database and stuff...  
echo $q;
  
$temp = mysql_query("SELECT MIS_id FROM materialinstorage WHERE MIS_id = '$q' "); 
echo $temp;

$row = mysql_fetch_array($temp);  
$id =$row['MIS_id'];  
echo $id;

document.getElementById("txtid").value="";应该是这句，这是什么错啊 

//跟你这句没关系的，这句只是设置文本框中的值为空
if (id.length==0){  //是这段的处理，主要是id.length这个没有定义，不懂这段做什么，是判断复选框是否被选？ 检查下这段
      document.getElementById("txtid").value="";
      return;
  }