本來想直接用Settings來獲取的，但是串列埠資訊類似 "\Device\Serial0"，死活獲取不了，用了轉義、反斜槓還是獲取不到，所以就放棄了，網上好像也沒有獲取成功的。
所以就用winapi來獲取了，參考了網上原始碼

cpp檔案
/*
    初始化串列埠
*/
void MainWindow::init_comm()
{
    QString path = "HKEY_LOCAL_MACHINE\\HARDWARE\\DEVICEMAP\\SERIALCOMM";
    QSettings *settings = new QSettings(path, QSettings::NativeFormat);
    QStringList key = settings->allKeys();
    QStringList comlist ;
    QStringList Baudlist ;
//    QStringList Paritylist ;
//    QStringList DataBitslist;
//    QStringList StopBitslist;
//    QStringList ControlFlowlist;

    comlist.clear();

    /* 取串列埠名 */
    for (int i=0; i<key.size(); i++) {
        comlist << getcomm(i, "value");
    }
    ui->ComPortcomboBox->addItems(comlist);

    ....
}

/*
    讀取鍵值
*/
QString MainWindow::getcomm(int index, QString keyorvalue)
{
    QString commresult;

    if (::RegOpenKeyEx(HKEY_LOCAL_MACHINE, TEXT("HARDWARE\\DEVICEMAP\\SERIALCOMM"), 0, KEY_READ, &hKey) != 0) {
            QString error="Cannot open regedit!";//無法開啟登錄檔時返回error
            return error;
        }//if(::RegOpenKeyEx(HKEY_LOCAL_MACHINE,subkey,0,KEY_READ,&hKey)!=0)

        QString keymessage;//鍵名
        QString message;
        QString valuemessage;//鍵值

        indexnum = index;//要讀取鍵值的索引號
        keysize = sizeof(keyname);
        valuesize = sizeof(keyvalue);

        if (RegEnumValue(hKey, indexnum, keyname, &keysize, 0, &type, (BYTE*)keyvalue, &valuesize) == 0) { //列舉鍵名和值
            for (int i=0; i<(int)keysize; i++) {
                message = keyname[i];
                keymessage.append(message);
            }

            for (int j=0; j<(int)valuesize; j++) {
                 if (keyvalue[j] != 0x00) {
                     valuemessage.append(keyvalue[j]);
                 }
            }

            if (keyorvalue == "key") {
                 commresult = keymessage;
            }

            if (keyorvalue == "value") {
                 commresult=valuemessage;
            }
        } else {
             commresult = "nokey";
        }

        RegCloseKey(hKey);//關閉登錄檔

        return commresult;
}
 

.h檔案

#include <QSettings>
#include <qt_windows.h>  
    HKEY hKey;
    LPCWSTR subkey;
    wchar_t keyname[256]; //鍵名陣列
    char keyvalue[256];  //鍵值陣列
    DWORD keysize,type,valuesize;
    int indexnum;