Is the Apple Watch series 1 compatible with the iPhone 7

HI

Yes, al models of Apple Watch (1st generation, Series 1 and Series 2) are compatible with iPhone 7.

To set up and use Apple Watch, you need an iPhone 5 or newer, running the latest version of iOS.

More information:

Set up your Apple Watch - Apple Support

Yes. First generation and second generation watches work with any iPhones, 5C and later.