Why my phone keep powered in off and on

iPhone 6 Plus is an outdated iPhone (9 september 2014),  which also affects the battery life.

That may be the cause of the problem.

Consider getting the battery replaced.

The battery can be replaced by Apple or an Apple Authorized Service Provider.

See this Apple article: https://support.apple.com/iphone/repair/service/battery-power

If you are going to replace the battery, read this Apple article: Get your iPhone, iPad, or iPod touch ready for service

https://support.apple.com/en-us/HT201557