When i tried to connect to mysql, there is a error:
QSqlDatabase: QMYSQL driver not loaded
QSqlDatabase: available drivers: QSQLITE QMYSQL QMYSQL3 QODBC QODB
This error generally means that Qt MySQL plugin is fine (as it is listed in available drivers), but you are a missing the MySQL dll (thus preventing the driver to load).
The way to fix it is to place libmysql.dll somewhere in your PATH, e.g. adding the MySQL installation folder to PATH, or copy libmysql.dll in the same folder you have your exe in.