下面是1个完整的例子，照着拷贝就可以了

test.aspx

<%@ Page Language="C#" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

    /// <summary>
    /// param1 param2 对应前面js传递来的参数。
    /// </summary>
    /// <param name="param1"></param>
    /// <param name="param2"></param>
    /// <returns></returns>
    [System.Web.Services.WebMethod]
    public static string GetDataTable(String param1, String param2)
    {
        return DataTable2Json(CreateDataTable(param1, param2));
    }

    public static System.Data.DataTable CreateDataTable(String param1, String param2)
    {
        System.Data.DataTable dataTable1 = new System.Data.DataTable("BlogUser");
        System.Data.DataRow dr;
        dataTable1.Columns.Add(new System.Data.DataColumn("UserId", typeof(System.Int32)));
        dataTable1.Columns.Add(new System.Data.DataColumn("UserName", typeof(System.String)));
        dataTable1.PrimaryKey = new System.Data.DataColumn[] { dataTable1.Columns["UserId"] };

        for (int i = 0; i < 8; i++)
        {
            dr = dataTable1.NewRow();
            dr[0] = i;
            dr[1] = "【孟子E章】" + i.ToString() + " 前端传递的参数的值分别是:" + param1 + ", " + param2;
            dataTable1.Rows.Add(dr);
        }
        return dataTable1;
    }

    public static string DataTable2Json(System.Data.DataTable dt)
    {
        StringBuilder jsonBuilder = new StringBuilder();
        jsonBuilder.Append("{\"");
        jsonBuilder.Append(dt.TableName.ToString());
        jsonBuilder.Append("\":[");
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            jsonBuilder.Append("{");
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                jsonBuilder.Append("\"");
                jsonBuilder.Append(dt.Columns[j].ColumnName);
                jsonBuilder.Append("\":\"");
                jsonBuilder.Append(dt.Rows[i][j].ToString());
                jsonBuilder.Append("\",");
            }
            jsonBuilder.Remove(jsonBuilder.Length - 1, 1);
            jsonBuilder.Append("},");
        }
        jsonBuilder.Remove(jsonBuilder.Length - 1, 1);
        jsonBuilder.Append("]");
        jsonBuilder.Append("}");
        return jsonBuilder.ToString();
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        //要传递的服务器控件的ID
        Page.ClientScript.RegisterClientScriptBlock(Page.GetType(), "js", "var div1 = '" + Panel1.ClientID + "';", true);
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>jQuery Ajax 调用后台方法返回 DataSet 或者 DataTable 的例子</title>
    <script type="text/javascript" src="jquery-1.8.0.min.js"></script>
    <script type="text/javascript" src="test.js"></script>
</head>
<body>
    <form id="form1" runat="server">
    <asp:Panel ID="Panel1" runat="server"></asp:Panel>
    </form>
</body>
</html>


test.js

window.onload = function () {
    $.ajax({
        type: "POST",
        url: window.location.pathname + "/GetDataTable",  /* 注意后面的名字对应CS的方法名称 */
        data: "{\"param1\":\"8888\",\"param2\":\"参数2\"}", /* 注意参数的格式和名称 */
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (result) {
            data = jQuery.parseJSON(result.d);  /*这里是否解析要看后台返回的数据格式，如果不返回表名则无需要 parseJSON */
            t = "<table border='1'>";
            $.each(data.BlogUser, function (i, item) { /* BlogUser是返回的表名 */
                t += "<tr>";
                t += "<td>" + item.UserId + "</td>";
                t += "<td>" + item.UserName + "</td>";
                t += "</tr>";
            })
            t += "</table>";
            $("#" + div1).html(t);
        }
    });
}