@jiang_peng, here is an example. The good thing here is this is a brand new file. Launched Fusion 360, started a sketch. Sketched the shape you see, threw a dimension at it and bang, it is not dragable and shows fully constrained. So, I saved it and shared it. Hopefully it shows the same behavior for you.
http://a360.co/1LtTVjx
If anybody cares to look (please) there is only one sketch. It is a D shape sketch. I should be able to move the arc left to right as the X dimension of the arc isn't constrained. I'm sure it is, but I can't see the constraint so I can't remove it. Thsi keeps happening and it is driving me crazy. My last file got so screwed up I had to start this one. litterally two minutes in and I am stuck in my tracks.
EDIT: I started a new file and followed the same steps and the problem repeats. Second one...http://a360.co/1PkZEyd
EDIT: Third time is a charm. http://a360.co/1GAFrgJ