新建一个阴影类，将这个类作为阴影背景
.h

class ShadowWindow : public QWidget
{
    Q_OBJECT

public:
    ShadowWindow(QWidget *parent=0);
    ~ShadowWindow();
protected:
    void paintEvent(QPaintEvent* event);
};

.cpp

#include "stdafx.h"
#include "ShadowWindow.h"

#include"PWindow.h"
ShadowWindow::ShadowWindow(QWidget *parent)
    : QWidget(parent)
{
    this->setWindowFlags(Qt::FramelessWindowHint|Qt::WindowStaysOnTopHint);
    this->resize(620, 420);//背景大于子窗口一点
    setAttribute(Qt::WA_TranslucentBackground);
    PWindow* pw = new PWindow(this);//子窗口
    pw->show();
}

ShadowWindow::~ShadowWindow()
{
}
void ShadowWindow::paintEvent(QPaintEvent* event) {
    int r = 170, g = 170, b = 170, a = 0;
    int x = 0, y = 0, w = this->width(), h = this->height();
    for (int i = 0; i <= 10; i++) {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing, true);
        painter.setPen(QColor(r - i * 15, g - i * 15, b - i * 15, a + pow(i,2.1)));
        //painter.drawRoundedRect(QRect(x + i, y + i, w - (i * 2), h - (i * 2)), 2, 2);
        painter.drawRect(QRect(x + i, y + i, w - (i * 2), h - (i * 2)));
        painter.end();
    }
}

如果子窗口覆盖了这个窗口，可以通过子窗口的拖动函数，调用move函数来移动父窗口，使得父窗口跟随子窗口移动，在子窗口要传入父窗口指针，子窗口类中定义指针接收父指针，这样就能操作父窗口。

QWidget* qw;//类中
qw = parent;//构造函数中
void PWindow::mouseMoveEvent(QMouseEvent* event){
    if (isMousePressed == true)
    {
        QPoint topLeftPoint = event->globalPos() - mousePosition;
        this->move(topLeftPoint);
        qw->move(this->x()-10,this->y()-10);//这里要做一下偏移，因为要流出阴影。
    }
}