My phone keeps turning itself off and on

@sayanmannas3830 Start off by changing the battery, then re-evaluate. Since iFixit does not yet have a guide for this, use this one for now. Check the charging board as well as the flex cable from the charging board to the main board. Make sure it is in good working condition (no tears, corrosion, etc.)

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.