Use OCI8 plugin of PYQT4 Can't not Log on to Oracle 8.1.7 Problem

I'm a new user. I tend to use python with pyqt4 to access the oracle database.
i put the OCI8.dll plugin in python lib directory.
In python i test the plugin:
database = QSqlDatabase.addDatabase("OCI8")
database.isValid()
it's return True

so i typed code:
database.setDatabaseName("orcl")
database.setUserName("system")
database.setPassword("manager")
database.open()
but it's return False
i get the lastError() it's shows me the message 'Unable to Logon'
someone who knows what's the problem?
oracle server is on another machine
i install oracle client on my own pc, and i can use the SQL Plus to connect the oracle
so i'm sure the oracle is accessable form my own pc.