As everyone has said, it depends on whether you're using iCloud Photos to synchronize your picture Libraries.
If you are using iCloud Photos, and you want the iPhone Library and the Mac Library to be completely independent, then Phil0124's suggestion to turn off iCloud on either the iPhone or the Mac, or both, is ideal. But then, if you don't want to synchronize your devices at all, you should consider giving up iCloud.
If you would like to empty the iPhone's Library but keep syncing new pictures to your Mac, then léonie's advice is the way to go. Keep your present Mac Library disconnected from iCloud, but make a an empty new one that is connected to iCloud to synchronize with the iPhone and collect the new pictures. See this:
Make a New System Library connected to iConnected to iCloud… - Apple Community
léonie's suggestion of making the new Library a favorites Library is great. I do that, too. So I can keep the synchronized iCloud Photos Library small, but keep all my old pictures.
The most important thing to do is to drag your current Mac Photos Library to an external drive, properly formatted as either APFS format or Mac OS Extended (Journaled) format. Before you mess around, you want to be sure you have a safe copy of the Library!