詳細代碼如下： 

//下拉框風格初始化
void comboBoxInit(QComboBox*box,int widthScroll=25,int height=30,bool bSheet=false);
//下拉框風格初始化
void comboBoxInit(QComboBox*box,int widthScroll,int height,bool bSheet)
{
    //顯示居中
    QLineEdit*lineEdit = box->lineEdit();
    if(lineEdit == 0)
    {
        lineEdit = new QLineEdit(box);
        lineEdit->setObjectName("lintEditAdptive");
        lineEdit->setReadOnly(true);
        lineEdit->setAlignment(Qt::AlignCenter);
        lineEdit->setFocusPolicy(Qt::NoFocus);
        QLineEdit::connect(lineEdit,&QLineEdit::selectionChanged,[=]{
            lineEdit->deselect();
            box->showPopup();
        });
        box->setLineEdit(lineEdit);
    }
    if(bSheet)//是否設置樣式
    {
        QString sheet = QString("QComboBox{\
            margin:2px;\
            background: rgba(0,0,0,0);\
            border: 1px solid #C0C4CC;\
            border-radius: 4px;\
            padding-left:%2px;\
            }QComboBox::drop-down {width:%2px;}\
            QComboBox QAbstractItemView {background-color:rgba(255, 255, 255, 255);}\
            QComboBox QAbstractItemView::item {height: %1px;}\
            QComboBox QAbstractScrollArea QScrollBar:vertical { width: %2px; }")
                            .arg(height).arg(widthScroll);

        box->setStyleSheet(sheet);
        box->setView(new QListView());

        //因為透明的，所以設置一個widget做背景
        QString objectName = box->objectName()+"Widget";
        QWidget*parent = qobject_cast<QWidget*>(box->parent());
        if(parent && parent->objectName()!= objectName) //  避免反復生成
        {
            QWidget* pWidget = new QWidget(parent); //創建一個widget
            pWidget->setObjectName(objectName);
            QRect rect = box->geometry();
            pWidget->setGeometry(rect);
            box->setParent(pWidget);//更改父窗口為pWidget
            box->move(0,0);
            pWidget->setStyleSheet("QWidget{background-color:rgba(225, 225, 225, 255);}");
        }
    }

    //設置字體
    QFont font = box->font();
    for(auto w:box->findChildren<QWidget*>())
    {
        w->setFont(font);
    }
    //下拉列表居中
    QStandardItemModel* model =  static_cast<QStandardItemModel*>(box->view()->model());
    if(model)
    {
        for(int i = 0; i < model->rowCount(); i ++)
        {
            if(model->item(i))
                model->item(i)->setTextAlignment(Qt::AlignCenter);
        }
    }

};

顯示文本要居中，需要重新設置LineEdit,用setLineEdit設置，樣式最好重新設置，否則比較不好看

下拉列表要居中，用QStandardItemModel 去設置，然后遍歷每一個下拉項，設置居中

int widthScroll 是下拉框滾動條寬度，也是顯示文本左偏移像素

int height是下拉列表子項的高度