System storage showing 427GB

Hi,

I am using Macbook Pro -

MacBook Pro (Retina, 15-inch, Mid 2015)

Sierra - 10.12.6 (16G2136)

I am getting alerts like system storage is too low...but when i check the storage details....

System - 427GB

Applications - 10GB

rest all 1-5GB only..

Any reason for system to show this much of storage...

Can i do disk clean up through recovery mode and reinstall the OS from Utility screen......does this require any backup drive or backup of OS..