感谢回复，不过 8 系列和 x 系列似乎换了一种启动方式，另外我的系统已经升级到 catalina 了，要打开 finder 回复，这里顺便贴一个老外的教程，万一有人能用的到。
How to Enter Recovery Mode on iPhone 8 Plus & iPhone 8
Be certain that you have a backup of your iPhone before using recovery mode. Failure to have an available backup may lead to permanent data loss.
1. Connect the iPhone 8 Plus, iPhone 8 to the computer with a USB cable
2. Open iTunes (Mac or Windows, or in macOS Catalina open Finder)
3. Press and release Volume Up on the iPhone
4. Press and release Volume Down on the iPhone
5. Press and continue holding the Power button until the iPhone 8 / Plus is in recovery mode
6. iTunes (or Finder) will show an alert message saying an iPhone has been found in Recovery Mode
Once the iPhone is in recovery mode it can be restored with iTunes or Mac Finder, or updated as usual.
You can also exit out of Recovery Mode by simply hard restarting the iPhone 8 Plus or iPhone 8.