It is not easy to take the exam, and the enrollment is small. Compared with most universities, it is more difficult to get a doctorate in Tsinghua, but it is easier to get a doctorate in Tsinghua than the college entrance examination, because fewer people will get a doctorate. If you want to take the exam, you should prepare well in advance. It doesn't matter where the undergraduate and master students studied. Many doctoral students in Tsinghua come from universities that are not so good.