为什么同一段JS代码，一个页面能用，另外两个不能用，谷歌报错，is not defind

<asp:TextBox ID="txtupload"  style="width:200px;float:left;" runat="server"></asp:TextBox>
            <iframe src="../uploud.aspx" runat="server" width="400" height="30" id="uploud" scrolling="no" frameborder="0" style="float:left;"></iframe>
            &nbsp;
            <span id="read" onClick="show()" class="preview">&nbsp;查看</span>&nbsp;&nbsp;
                <a id="del" style="color:Red;" onClick="del();">删除</a>
            &nbsp;&nbsp;&nbsp;此图片为单页的缩略图，不会在单页的详细内容页显示。
            <div id="img" style="display:none;">
                    <br />
                <img id="showimg" width=120 height=100 />
                </div>
                <script type="text/javascript" language="javascript">
                    function del() {
                        var uploud = $("#txtupload");
                        if (uploud.val() == "") {
                            alert("请选择文件");
                            return;
                        }
                        else {
                            jQuery.ajax({
                                type: "get",
                                url: "../js/ajax.aspx",
                                dataType: "text",
                                contentType: "application/json;charset=UTF-8",
                                data: {
                                    imgurl: escape(uploud.val())
                                },
                                success: function (result) {
                                    alert(result);
                                    uploud.val('');
                                }
                            });
                        }
                        document.getElementById("img").style.display = "none";
                        document.getElementById("read").innerHTML = "查看";
                    }

各位前辈，有谁能告诉我原理么？