QT5: Failed to load psql driver in windows

I want to load qt5 psql driver in windows 7.

I've load the library as this way:

qDebug() << QCoreApplication::libraryPaths ();
QString driverName = "QPSQL";
QSqlDatabase::addDatabase(driverName,"A connection to postgres");

And the output shows:

("C:/QT/Qt5.1.0/5.1.0/msvc2010/plugins", "D:/xxx/build-xxx-Desktop_Qt_5_1_0_MSVC2010_32bit-Release/release")
QSqlDatabase: QPSQL driver not loaded
QSqlDatabase: available drivers: QSQLITE QODBC QODBC3 QPSQL QPSQL7

And in directory: C:/QT/Qt5.1.0/5.1.0/msvc2010/plugins/, there does have sqldrivers subdirectory and qsqlpsql.dll, qsqlpsqld.dll, qsqlpsqld.pdb in the subdirectory.

I load the psql library successful in ubuntu.

and my .pro file is mainly like this:

QT += core gui sql concurrent

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

unix {
LIBS += -lpython2.7 
QMAKE_CXXFLAGS += -std=c++0x
}

win32 {   
LIBS += \
    C:/Python27/libs/python27.lib 
}

I can't find the reason why it failed.

Do I need to add libpqdll.lib in .pro file? But I didn't use LIBS += -lpq in ubuntu?

BTW: I failed to build libpq in windows 7, using visual studio 2010. But this is another problem.

The question may be easy. But I've tried a wholely day.