确定了设置图标的格式，但是设置任务栏图标去出现了问题，死活显示不出来于是做了一个小的例程用来测试

例程

vim untitled1123.pro

QT       += core gui webenginewidgets

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    mainwindow.h

FORMS += \
    mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

vim mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtWebEngineWidgets>
#include <QIcon>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QWebEngineView *view = new QWebEngineView;
    QUrl url = QUrl::fromUserInput("http://www.baidu.com");
    view->load(url);

    ui->gridLayout->addWidget(view,0,0);
}

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

vim main.cpp

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    w.setWindowIcon(QIcon("./task.ico"));
    return a.exec();
}

通过例程，发现可以设置啊，用的当前路径，那就奇怪了。

经过一番尝试，发现需要通过资源文件才能够正确显示任务栏图标，奇了怪了。。。
添加资源文件如下，在pro文件中：

RESOURCES += \
    source.qrc

添加了qrc资源文件，就不需要添加ICO了

unix {
#    ICON = platform/Kylin/Resources/task.ico
}

c文件中：

#ifdef Q_OS_MAC
    //设置程序图标
    SingleApplication::setWindowIcon(QIcon(":/image/platform/MacOS/Resources/Icon.icns"));
#else
    SingleApplication::setWindowIcon(QIcon(":/image/platform/Kylin/Resources/task.ico"));
#endif

qrc文件中：

<RCC>
    <qresource prefix="/image">
        <file>platform/MacOS/Resources/Icon.icns</file>
        <file>platform/Kylin/Resources/task.ico</file>
    </qresource>
</RCC>

自此，才可以正确的显示任务栏图标。

参考：为QT生成的应用程序加图标 \QWindow::setWindowIcon()