I read from here (a Chinese website) that Martin Luther called John Calvin a son of the Devil.
Now I have never seen this quote anywhere. Did Luther say something like this to Calvin? If yes, why?
I cannot find a reference that he did, but it's possible he may have on a specific issue. From my research there is no evidence that he every called Calvin "the son of the Devil" and in fact those are Calvin's words, not Luther's.
Let's get some facts.
Calvin and Luther were contemporaries, both living in Europe during the early to mid 1500s. However, Luther was born in the 1480s and Calvin was born in 1509. Also, while both men were fluent in Latin, each wrote the majority of their works in their native tongues. Calvin in French, Luther in German.
However, it seems, while they had significant theological differences in many areas. Luther appreciated Calvin and Calvin appreciated Luther.
I think I may (however) have found the source of the quotation you refer to. These were not Luther's words, but Calvin's own:
This was a discussion of Luther's possible reaction to Calvin's views on the Lords Supper which were a marked contrast both from Rome's view but also Luther's. This was an acknowledgement both to how indebted he felt to Luther, and his strength of will that he was willing to disagree with Luther and potentially face his notorious wrath.
Likely this fear was motivated by the heated debates between Luther and Zwingli over the presence of Christ in communion.
There is little (if any) evidence that Luther ever addressed Calvin's position on the Lord's Supper specifically. However, due to the debates with Zwingli and Calvin's acknowledgement that his position was more similar to Zwingli's than Luther's he recognized that he certainly put himself in position to be the target of heated name calling or other insults from Luther.
(1) Martin Luther and John Calvin