Mac storage basics · Photos, music and video
Can I delete photos from my Mac but keep them on my iPhone?
Short answer: Only if iCloud Photos is off. With it on, deletions sync to every device — use Optimise Mac Storage instead to keep the Mac's copies small.
Only if the two libraries aren't synced. With iCloud Photos on, the Mac and iPhone share one library: deleting on the Mac deletes on the phone too. In that case, turn on Optimise Mac Storage in Photos' iCloud settings instead, so the Mac keeps smaller versions and downloads full ones when needed.
Without iCloud Photos, the Mac's library is separate — photos you imported can be deleted on the Mac without affecting the phone. More on iCloud Photos options: iCloud & iCloud Photos.