1、可能是客户端文件配置有问题，listener.ora、tnsnames.ora

2、把两个文件拷贝到客户端安装目录的$ORACLE_HOME\client_1\network\admin下面

3、在tnsnames.ora添加如下内容：

ORCL =

  (DESCRIPTION =

    (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))

    (CONNECT_DATA =

      (SERVER = DEDICATED)

      (SERVICE_NAME = orcl)

    )

  )

4、在listener.ora添加如下内容：

LISTENER =

  (DESCRIPTION_LIST =

    (DESCRIPTION =

      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))

      (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))

    )

  )

5、打开pl\sql工具，输入用户名：scott；密码：tiger；数据库是orcl；登录模式：Normal

6、搞定。！！