QTextDocument* doc = ui->edtCountryIntroduce->document();

for(QTextBlock it = doc->begin(); it != doc->end(); it = it.next())
{
    QTextCursor textCursor(it);
    QTextBlockFormat textBlockFormat = it.blockFormat();
    //set line height
    textBlockFormat.setLineHeight(24,QTextBlockFormat::FixedHeight);  
    textCursor.setBlockFormat(textBlockFormat);
    ui->edtCountryIntroduce->setTextCursor(textCursor);
}

    QTextCursor textCursor = ui->textEdit->textCursor();
    QTextBlockFormat textBlockFormat;
    //set line height
    textBlockFormat.setLineHeight(24,QTextBlockFormat::FixedHeight);  
    textCursor.setBlockFormat(textBlockFormat);
    ui->textEdit->setTextCursor(textCursor);