My phone display is shaking,why?

You may have a hardware problem

First try a forced restart

https://support.apple.com/en-gb/guide/iphone/iph8903c3ee6/ios

If that does not help the  next procedure would be to backup then restore your device to factory, and setup the device without restoring a backup, to see if the issue is replicated. 

Then restore the backup. 

If you need to send the phone in for repair then backing up and restoring to factory would be needed anyway.

Here is a guide for those steps

Backup and restore (I recommend the computer method if you have one)

(1)How to back up your iPhone, iPad, and iPod touch – Apple Support

Remove SIM PIN if you have set one

(2) Restore your iPhone, iPad, or iPod to factory settings – Apple Support

Set up as new from the Hello Screen without restoring backup. Test if the problem still exists

If the problem goes away in the newly set up phone then suspect settings or apps, and in particular VPNs and so-called security apps which should never go near an iphone.

If the problem remains then arrange a repair including details of these tests.

https://support.apple.com/en-gb/iphone/repair/service

(3) When your phone comes back Restore your iPhone, iPad, or iPod touch from a backup