Hello Min. (:
Here’s how to fix your Chromebook screen:
Perform a [https://support.google.com/chromebook/answer/3227606?hl=en|hard reset] of the Chromebook. Sometimes, this will remove these lines since you are resetting the hardware. or,
[https://support.google.com/chromebook/answer/1080595?hl=en|Restore your chromebook]. This will reinstall Chrome OS which will eliminate the issue if it is a software conflict. or,
Take your chromebook to a repair store.
Hope this helps. (: