Of all the levels of education, doctoral education is the most internationalised. By selecting one key indicator (the proportion of international students among a country's doctorate recipients), the article presents an analysis of PhD students' international mobility. After the collapse of the Soviet Union and the end of the Cold War in the early 1990s, many barriers to the international mobility of PhD students were removed, leading to an even larger flow of students. From 2000 to 2012, the international mobility of PhD students reached a new peak, with a significant increase in the percentage going to Japan, France, Canada, Germany, the United Kingdom and Australia. Nowadays, China has become the largest source of international doctoral students. In that process, a number of excellent students go abroad to study for foreign doctoral degrees. On one hand, the loss of excellent students is harmful to China's PhD training system, but on the other hand, it is a great opportunity for China to change brain drain into brain gain by making full use of the knowledge diaspora scattered around the world.