Hi Zhao Lu,
Have you tried following the official guide to change database here:
http://docs.jboss.org/jbpm/v5.4/userguide/ch.installer.html#d0e609
I wrote an article once but this is for using MS SQL Server
https://community.jboss.org/wiki/SetUpJBPM54FinalInstallerToUseMSSQLServer2008UsingJTDS
Regards,
Thomas Setiabudi