Chen Lanbin is China’s first ambassador to the United States, honest and upright, using evidences to safeguard the human rights of Chinese workers in the Americas, and actively negotiating the Denver anti-Chinese incident. As a subordinate of Chen Lanbin, Cai Xiyong chose to translate the US Federal Constitution, which was due to not only his personal interest but also the influence of Chen Lanbin as a patron. Based on Lefevere’s “patronage” theory, the the reasons why Cai Xiyong chose to translate the US Federal Constitution, why there existed linguistic deviations and deformations between Cai’s translation and the original text and why Cai’s translation did not catch people’s enough attention in modern China history, are examined from the three aspects of patronage: ideological, economic and status factors. On ideological factor, Both Chen Lanbin and Cai Xiyong adhered to the concept “Chinese learning as substance, western learning for application”; on economic factor, Cai Xiyong was appreciated by his leader and patron Chen Lanbin and got a salary from the Qing Dynasty government; on status factor, Cai Xiyong was recognized by his leader and patron Chen Lanbin and became an official through the Westernization Movement. Influenced by the patron Chen Lanbin, the purpose of Cai Xiyong translating the US federal Constitution is to handle the Denver anti-Chinese incident, and not to reform China’s political and legal system, transform the superstructure and overthrow the feudal rule of the Qing Dynasty government.