This story is mentioned in the book Ithbat al-Hodat, compiled by al-Hurr al-'Amili (vol. 4, p. 28).
The gist of the story is a king in China killed his daughter and a member of his court, regretted it, and sought advice from his court on how to bring them back to life. One of the viziers said there is a man in Medina named al-Hasan ibn 'Ali who could do this. The king told him he had a month to bring al-Hasan there (even though it was about a 6 month journey from China to Medina). The vizier prayed intensely, and, while he was praying, he heard a voice saying "Rise". He rose and found Imam al-Hasan (A) there. Imam al-Hasan (A) passed his hand over the two people who had been killed and prayed that Allah revive them, and they came back to life. Then the king married his daughter to Imam al-Hasan (A).
I am not aware of a chain of narration for the story and so it is not possible to say from that angle whether it is authentic or not. Also, I am not aware of any historical evidence that Imam al-Hasan (A) went to China. So, it is possible that this story is not historically true.
However, a number of hadith indicate that the Imams had the ability to miraculously travel from place to place instantaneously and perform karamat through the permission of Allah, and so that main idea in and of itself is reasonable and supported by other texts, even if this particular story might not be true.