I want to know the top 50 universities in the US that have offered the
most P.hDs. in Mechanical Engineering to students who come from China.

that question would take way more than $10 to answer.

instead, consider:
http://www.usnews.com/usnews/edu/grad/rankings/eng/brief/engrank_brief.php

It is a list of the top 50 Engineering schools in the USA.  Since I
studied Civil Engineering, I am confident that most of these top
schools will naturally draw many good students from China.

good luck!