Nexus 5x No Command screen I was attempting to upgrade the os to 7.1.1, but after downloading, installing the os and restarting the device, the screen shows "No command" with a dead android icon.

The "No command" screen is like this: 



I followed this link to install TWRP instead, everything works fine, but at the end the screen still shows "No command", how could I escape from it?

http://www.ibtimes.co.uk/how-root-google-nexus-5x-install-twrp-recovery-1525518

Actually, I tried this 
http://forums.androidcentral.com/droid-razr-m/332350-wipe-cache-no-command.html, but still in vain
It seems like TWRP wasn't installed. You still have the stock recovery.

As you've got to the "no command" screen, try this:  Hold down the power button for 3-5 seconds and then press and release the Volume up button.