Hi Bumper, my comments in blue below:
HI elcpu
thank you for your reply, but it was not addressing my question. I know that once I activated icloud photo library, the camera roll and photo stream will be 'merged'. But in the same time I can not delete my photo in my iphone without deleting the photo in my icloud library. That is true. The article I linked in my previous post states: "When you delete a photo or video from your Photos app, it will also delete from your iCloud Photo Library and iCloud storage. You can recover your photos and videos from the Recently Deleted album for 30 days."
I bought icloud space, and want to retain all my photo in the cloud, but if i delete photos in my iphone, the photos in the cloud will also be deleted. What is the point I bought icloud space? That is the way Apple engineers designed iCloud Photo Library. The purpose was to share all your photos across all your devices without the need to download them to each. It does not work the same as DropBox or PhotoBucket where one is independent of the other. You might want to consider those options if your desire is just to keep photos on the cloud and accessing them manually, not automatically. Here is a different Apple article that may answer your questions.
iCloud Photo Library FAQ - Apple Support
In particular, look at the section on: Can I use iCloud Photo Library to save space on my device? "If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever. All of your original, full-resolution photos and videos are stored in iCloud while device-size versions are kept on your device. You can download the original photos and videos over Wi-Fi or cellular when you need them."
so, my question is: how to delete photo in my iphone without deleting tho photos in my icloud library. Unfortunately the answer is you cannot, not a feature of iCloud P. L. As I mentioned, other 3rd party cloud storage may work better for you if that is what you want, the drawback being that you would have to download the photos each time you want to look at them on any device.
Hope this clarify my problem. Yes it did, thanks....
Once again thanks! My pleasure. Not the answer you wanted perhaps but hopefully it will clarify what iCloud P.L. can do. I suggest reading the FAQ in detail, I think they will explain what can and cannot be done with iCloud P. L.
Good luck...