Cui Tiankai, the longest-serving Chinese ambassador to the United States, will leave his post and return to Beijing soon.

Cui worked as an interpreter with the Chinese delegation to the UN at the General Assembly in 1979, when China and the U.S. forged their diplomatic relations.

He had been vice minister of foreign affairs and ambassador to Japan before being appointed as China's top envoy to the U.S.

Since embarking on a journey to serve "as China's ambassador to the United States in April 2013, I have been working on this post for more than eight years," Cui said in a farewell letter on June 21.

Cui also said the China-U.S. relationship now stands at a "critical crossroads." As the U.S. policy toward China undergoes a new round of restructuring, it faces the choice between engaging in dialogue and cooperation, or confrontation and conflict.