更新
您的查询中有一个错误（有时您需要在另一个类似mysql clis或online的环境中调试您的查询）http://sqlfiddle.com)

SELECT username,password from users where username = :username AND password = :password

而不是

SELECT username and password from users where username = :username AND password = :password

更正的答案

void MainWindow::on_loginBtn_clicked() {

  QSqlDatabase db;

  db = QSqlDatabase::addDatabase("QMYSQL", "MyConnect");
  db.setHostName("localhost");
  db.setUserName("root");
  db.setPassword("");
  db.setDatabaseName("qtregister");

  QString username = ui - > loginEdit - > text();
  QString password = ui - > loginPassword - > text();

  if (db.open()) {

    QSqlQuery query(db);

    query.prepare(QString("SELECT username , password from users where username = :username AND password = :password"));
    query.bindValue(":username", username);
    query.bindValue(":password", password);

    if (!query.exec()) {
      QMessageBox::information(this, "Failed", "Error in executing query");

    } else {
      while (query.next()) {
        QString usernameFromDB = query.value(0).toString();
        QString passwordFromDB = query.value(1).toString();
        qDebug() << usernameFromDB << passwordFromDB;
        if (usernameFromDB == username && passwordFromDB == password)
          QMessageBox::information(this, "Success", "Login Success");
        else
          QMessageBox::information(this, "Failed", "Username or password error");
      }
    }
  } else {
    QMessageBox::information(this, "Not Connected", "Not Conneced Success");
  }
}