Translation:This is Doctor Li.
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Yes, it is wrong. The reason is as follows.
We use "The" when we want to make a noun definite. e.g. "This is a doctor" indicates that you are referring to some doctor, but know nothing else. "This is the doctor" means that you are referring to a specific doctor you had previously mentioned. e.g. "The doctor who lives near my house.." and now you are telling us that "This is the doctor", whom I had told you about earlier..
Since Doctor Li is a specific person.. not any random doctor, so the word is already definite. You should not and cannot add the because it sounds weird that you are trying to make something definite which is already definite.
Hope this helps.
P.S. If you know multiple Doctor Lis, then maybe you are pointing out that this is a specific Doctor Li; in which case your sentence may sound okay. But that's a rare exception... In general saying "the Doctor Li" will sound weird and wrong as I explained above.