参考：http://blog.csdn.net/augusdi/article/details/8865541

代码如下：

注意，要在ui界面上放置一个“Vertical Layout”控件，调整到合适大小

#include <QtWidgets/QMainWindow>
#include <QPaintEvent>
#include <QTimer>
#include <QPainter>
#include <QPixmap>
#include <QLabel>
#include <QImage>
#include <opencv.hpp>
#include "ui_mainwindow.h"
using namespace cv;
class mainwindow : public QMainWindow
{
	Q_OBJECT

public:
	mainwindow(QWidget *parent = 0);
	~mainwindow();
public slots:
	void updateImage();
private:

	QTimer theTimer;
	Mat srcImage;
	VideoCapture videoCap;
	QLabel *imageLabel;

	Ui::mainwindowClass ui;

protected:
	void paintEvent(QPaintEvent *e);
};

mainwindow::mainwindow(QWidget *parent)
	: QMainWindow(parent)
{
	ui.setupUi(this);
	connect(&theTimer, &QTimer::timeout, this, &mainwindow::updateImage);
	//从摄像头捕获视频
	if(videoCap.open(0))
	{
		srcImage = Mat::zeros(videoCap.get(CV_CAP_PROP_FRAME_HEIGHT), videoCap.get(CV_CAP_PROP_FRAME_WIDTH), CV_8UC3);
		theTimer.start(33);
	}
	//设置显示视频用的Label
	imageLabel = new QLabel(this);
	ui.verticalLayout->addWidget(imageLabel);
}

mainwindow::~mainwindow()
{

}

void mainwindow::paintEvent(QPaintEvent *e)
{
	//显示方法一
	QPainter painter(this);
	QImage image1 = QImage((uchar*)(srcImage.data), srcImage.cols, srcImage.rows, QImage::Format_RGB888);
	painter.drawImage(QPoint(20,20), image1);
	//显示方法二
	QImage image2 = QImage((uchar*)(srcImage.data), srcImage.cols, srcImage.rows, QImage::Format_RGB888);
	imageLabel->setPixmap(QPixmap::fromImage(image2));
	imageLabel->resize(image2.size());
	imageLabel->show();
}

void mainwindow::updateImage()
{
	videoCap>>srcImage;
	if(srcImage.data)
	{
		cvtColor(srcImage, srcImage, CV_BGR2RGB);//Qt中支持的是RGB图像, OpenCV中支持的是BGR
		this->update();	//发送刷新消息
	}
}