现在达到这样的效果了：
如果能够区分根节点和叶子节点，那么我就可以将根节点的图片换成不带连接点的＋号了。
QSS代码如下：
QTreeView::branch:has-siblings:!adjoins-item {
margin-left: 6px;
border-image: url(:/branch-vline.png);
}
QTreeView::branch:!has-children:has-siblings:adjoins-item {
image: url(:/branch-more.svg);
margin-left: -45px;
background-image: url(:/branch-single.svg);
background-position: right;
background-repeat: no-repeat;
}
QTreeView::branch:!has-children:!has-siblings:adjoins-item {
border-image: none;
image: url(:/branch-end.svg);
margin-left: -45px;
background-image: url(:/branch-single.svg);
background-position: right;
background-repeat: no-repeat;
}
QTreeView::branch:has-children:adjoins-item:open {
border-image: none;
image: url(:/branch-more.svg);
margin-left: -45px;
background-image: url(:/branch-open.svg);
background-position: right;
background-repeat: no-repeat;
}
QTreeView::branch:has-children:adjoins-item:closed {
border-image: none;
image: url(:/branch-more.svg);
margin-left: -45px;
background-image: url(:/branch-closed.svg);
background-position: right;
background-repeat: no-repeat;
}
QTreeView::branch:has-children:adjoins-item:open:first {
background-image: url(:/branch-root-open.svg);
}
QTreeView::branch:has-children:adjoins-item:closed:first {
background-image: url(:/branch-root-closed.svg);
}
