I want to buy charger for Mac Pro ,but I don't know the exactly which one it fits my Mac pro

According to About Mac power adapters - Apple Support , a mid 2015 15" MacBook Pro uses the following:

85W MagSafe Power Adapter with MagSafe 2 style connector

• 15-inch MacBook Pro models introduced in 2012 through 2015

That is this one in the Apple online store in the U.S.: Apple 85W MagSafe 2 Power Adapter - Apple

What model year is your 15" MacBook Pro? That is available at Apple menu > About this Mac. With that information you can use the following to identify the proper power adapter: About Mac power adapters - Apple Support