Please find my sample program,

Dialog:ialog(QWidget *parent) :
QDialog(parent),
ui(new Ui:ialog)
{
ui->setupUi(this);
this->addItems();
connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(slot_pushbuttonpressed()));
}

Dialog::~Dialog()
{
delete ui;
}

void Dialog::addItems()
{
for(int i = 0 ; i < 500; i++)
{
QString data = "Test_"+QString::number(i);

ui->comboBox->addItem(data);
}
}

void Dialog :: slot_pushbuttonpressed()
{
ui->comboBox->clear();
for(int i = 0 ; i < 5; i++)
{
QString data = "Test_"+QString::number(i);

ui->comboBox->addItem(data);
}
}