In PLSQL Developer, the complete server of Oracle9i is installed on the machine that modifies the Oracle Home setting method, and an Oracle10g client is also installed, in this way, my machine has two Oracle Home instances at the same time. During the previous connection, PLSQL Developer uses 10 Gb Home by default, so the connection fails. In PLSQL Developer, modify the Oracle Home setting method: the system prompts that the user name and password are entered, and the password is canceled. In this way, go to the PLSQL Developer main interface, and choose Tools-Preferences (Tools-Preferences). In the setting window, select Oracle-Connection (oracle --- Connection). In the corresponding settings window, find Oracle Home (Home directory name), select OraHome92, save the settings, and restart PLSQL Developer, then you can log on normally. Of course, if you need to re-use the local Net service of a 10 Gb database, you need to re-Change it back to OraClient10g_home. In short, when the computer has multiple home, you need to modify the home in pl SQL.