Translation supplement is one of the important methods of translation. This article takes Watson’s English translation of Shiji and Mozi as an example, comparing them with Yang’s version of Shiji and Mei’s version of Mozi and finding that the law of supplement in English translation of classics namely includes cohesion supplement, explanation supplementation and clause supplementation. Moreover, it is found that the supplemented translation is often more readable and easier for foreign readers to understand Chinese culture. On this account, this article explores the application this law to English translation of classics, and tries to combine it with the teaching of English translation of classics in colleges and universities in order to promote the development of English translation of Chinese classics.