objTable.rows[Row_Count].cells.length,这里出问题了,因为你删除了一列后你的objTable.rows[Row_Count].cells.length长度就会变短,所以
你需要重新取得它的列长度
改了下:    function UniteRow()
    {
alert(Row_Count);
        var objTable=document.getElementById(TableName);
        for(var j=1;j<objTable.rows[Row_Count].cells.length;j++) //
        {

                objTable.rows[Row_Count].cells[0].innerHTML +=objTable.rows[Row_Count].cells[j].innerHTML;            
                DelTd(Row_Count,j);            
                objTable.rows[Row_Count].cells[0].colSpan=parseInt(objTable.rows[Row_Count].cells[0].colSpan) + 1;
                alert("DelTd("+Row_Count+","+j+")+行"+Row_Count+"列"+j+"成功被删除!");
         j--;
}
        document.getElementById("DelRow").disabled = "disabled";
        document.getElementById("UniteRow").disabled ="disabled";
    }你的思路挺好,这代码也不错,呵呵收了