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。！！