One of my friend used to bend his thumb backwards. I wonder, is there any word in English to call the nature of thumb bending backwards?
I believe you are referring to "Hitchhiker's thumb"
In addition to the term double-jointed, you could use the term loose joints. The medical community uses the word hypermobility to explain this condition, which includes the ability to "bend your thumbs to your forearms."
Like Barrie, I don't know of any term related specifically to the thumb, but I did find more than 9,000 hits on Google for "hypermobility of the thumb," and 1,200 for "hypermobile thumb," including some from medical literature.