感謝簡友提供的思路 http://www.lxweimin.com/p/c59d55609a98

不然一直以為是要在collectionViewCell里面設置灰色邊框！

現在直接在vc設item的size為
let kHomeItemW = kScreenW/3 - 5
//行間距，此屬性大小決定上下兩個cell的間距
layout.minimumLineSpacing = 1
//minimumInteritemSpacing：項間距，此屬性設置為0，然后通過控制itemSize的大小來控制左右兩個cell的間距
layout.minimumInteritemSpacing = 0
//灰色
collectionView.backgroundColor = RGBA(r: 233, g: 233, b: 233, a: 1)
然后把
collectionViewCell設為白色就可以了!

 // MARK:- 初始化 collectionView
    lazy var collectionView:UICollectionView = { [unowned self] in
        let layout = UICollectionViewFlowLayout()
        layout.itemSize = CGSize(width: kHomeItemW, height: kHomeItemH)
        //行間距，此屬性大小決定上下兩個cell的間距
        layout.minimumLineSpacing = 1
        //minimumInteritemSpacing：項間距，此屬性設置為0，然后通過控制itemSize的大小來控制左右兩個cell的間距
        layout.minimumInteritemSpacing = 0
        layout.sectionInset = UIEdgeInsets(top: kItemMargin, left: kItemMargin, bottom: kItemMargin, right: kItemMargin)
        // frame 在makeContraints設置
        let collectionView = UICollectionView(frame: CGRect.zero, collectionViewLayout: layout)
        collectionView.backgroundColor = RGBA(r: 233, g: 233, b: 233, a: 1)
        collectionView.register(LotteryHomeCell.self, forCellWithReuseIdentifier: identify)
        collectionView.layer.cornerRadius = 5
        collectionView.layer.masksToBounds = true
        collectionView.isScrollEnabled = false
        collectionView.showsHorizontalScrollIndicator = false
        collectionView.dataSource = self
        collectionView.delegate = self
        collectionView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
        collectionView.layoutIfNeeded()
        return collectionView
    }()