I'm assuming you would like a graphic depiction of the "flattened" tube along with dimensions to bends and so forth. In case all you want is the length for documentation, you can determine the length by using the Inquire Element and/or the Total Length commands.
the Inquire Element Jef did mention will only give you the
length of the part as is.
[...
First ,i dont konw how to unbend a tube(round or square)
[...]
we too ;-) It's not possible in SE to unbend a tube. Maybe
that special software (external) can do that
It's true SE cannot "straighten" a tube. Sometimes companies that sell machinery such as CNC pipe benders will offer proprietary software to come with it. You may be able to purchase this seperately.