The screen of my Acer laptop shakes after booting.

Please explain what you mean by shaking? Does the screen physically shake? Does the image on the screen shift around? Or do you mean it flashes?

Can you post a video/photo of what is happening?

Is it under warranty? If so, take it to an authorized repair place to have it fixed.

The image on the screen flashes

Is it just the backlight that flashes? Or does the screen stay lit? Does it stop flashing after the computer is done starting up? If you move the hinge, like open/close the laptop does it reproduce the issue? What about pinching the screen near the hinges (not the screen itself, but the bezel/frame)?

Can you post a video/photo of what is happening?

Is it under warranty? If so, take it to an authorized repair place to have it fixed.