Hi!! We would like to know how to get to the Changbaishan from Dandong and if it's possible to do it in one day since we have our hotel there.

I'm afraid you may not reach there in one day by train or public bus. The train from Dandong to Baihe takes nearly 18h and after that, you need to spend half an hour to rent a car to the destination. If you really wish to reach there, try to rent a car, but it will be very expensive.