Thank you very much!
I look up the dictionaries again and find it.
Now I think I make it clear.
When opposite is a preposition, it can be used as "be opposite sth" just as you said. Since this is quite strange to me, I somewhat ignore this usage in dictionary(quite sorry for that).
When opposite is a noun, it can be used as " the opposite of/to sth "
When it is an adjective, it can also be use as "be opposite to "(although many examples on the internet, I can't find this usage in dictionary.)
More details can be found in this website : http://english.stackexchange.com/que...to-or-opposite
Here I quote the answer of SG324:
"For this specific scenario, the usage of "opposite to" and "opposite" is indistinguishable in resulting meaning.However, they cannot be used interchangeably in all cases.
"Opposite" is utilized as an adjective in:
Their house is opposite to the Red Cross Hospital.While it is utilized as a preposition in:
Their house is opposite the Red Cross Hospital.The divergence lies in how "opposite" as an adjective can accept an argument before it takes on the prepositional phrase headed by "to" (PP-to).
So while we can say:
The dice are opposite [in color] to the marble.We cannot say:
*The dice are opposite [in color] the marble.But the following statements tend to be construed in the same manner:
The rook is opposite to the knight.This suggests that the prepositional variant, when contrasted to the adjectival variant, is limited to having the same implied meaning as "opposite in position to". So, in conclusion, "opposite" as the head of the adjectival phrase that dominates PP-to has the potential to be applied with a greater scope.
The rook is opposite [in position] to the knight.
The rook is opposite the knight.
I think that about covers it.