Hi Lei - A few things: does this happen with any model or only specific ones? Since backfaces are not colored in the default display modes, please either attach your custom mode or describe the changes to one of the default modes. Finally, please run the Rhino SystemInfo command and post the result here.
-wim
When I open a new file(no attchment nor copy-in), the extrusion is all fine. No back surfaces(as marked as purple colour). BUT, here is the file 191104_error file.3dm (909.9 KB) when I have these problems. Either you attach this file into a new file or create an extrusion geometry in this file, the problem occurs.
It looks like you are running a cracked version of Rhino and while that probably isn’t what is causing this, you might as well try with a legitimate copy.
You are running Rhino 6.13 - please update to the current 6.19. Also, your display drivers are getting somewhat old - Driver Date: 11-29-2018 - please try updating these as well.
-wim