Thank you , LyKurg
Here is my program:

void cgStaticTextElement::slot_onSetTextAlign( FParam* param )
{
int alignFlag = param->getValue<int>();
QTextOption option = document()->defaultTextOption ();
Qt::AlignmentFlag flags((Qt::AlignmentFlag)(alignFlag));
option.setAlignment ( flags );
document()->setDefaultTextOption ( option );
notifyReCache(this);
}

the option in this,have a right value ,like Qt::AlignRight , but the text in the QGraphcisTextItem 's Alignment not change.
If I have modif my program like this :

void cgStaticTextElement::slot_onSetTextAlign( FParam* param )
{
int alignFlag = param->getValue<int>();
QTextOption option = document()->defaultTextOption ();
document()->setTextWidth(200); //At here give a fixed value
Qt::AlignmentFlag flags((Qt::AlignmentFlag)(alignFlag));
option.setAlignment ( flags );
document()->setDefaultTextOption ( option );
notifyReCache(this);
}

the Alignment of the QGraphcisTextItem have changed...

I don't know why ? Would you give me a reason? Thank you .