I have used solution of musicamante, thank you a lot!

Firstly I have to decrypt encrypted database to a QByteArray, then I writing decrypted data to aQTemporaryFile .

Next, copying database content that file to a QSqlDatabase stored in :memory: . After that, I overwrite temporary file`s content by zeroes(to make deletiong more safer) and deleting it.

If I want to write back changed data I need to create QTemporaryFile and copy :memory: database to it, afterwad copy content of file to QByteArray , ecnrypting it and writing to encrypted database file.