He was known for his mastery of sources for Chinese history and philosophy, his ability to synthesize them on a wide range of topics, and for his advocacy for a new Confucianism.

Yu's father, who had studied at Harvard, taught history in Tianjin, and at the start of the second Sino-Japanese War sent him to live with his aunt from 1937 through 1946 in rural Anhui province, where they would be safe from Japanese invasion.

The founders of New Asia College, which Yu joined as a student, were staunchly anti-Communist, rejected the iconoclastic New Culture Movement but did not see Western liberal thought as the alternative.

[1] While still in Hong Kong, Yu started to write books and pamphlets in Chinese commenting on the problems of intellectuals and democracy in the People's Republic.

[7] He was particularly tenacious over the years in presenting the achievements of Chen Yinke (1890–1969), the greatest modern scholar of Tang dynasty China, who was at first supported and then hounded to death by the revolution.

His Harvard PhD thesis was published as Trade and Expansion in Han China; a Study in the Structure of Sino-Barbarian Economic Relations (Berkeley: University of California Press, 1967).

Scrupulous and thematically relevant monographs, mostly published in Chinese, explored the role of intellectuals, especially early modern moral and political critics such as Fang Yizhi (1611–71), Dai Zhen (1723–77), and Zhang Xuecheng (1738–1801), who had been neglected in earlier scholarship.

Yu also mastered the scholarship around Honglou Meng, the novel known in English as Dream of the Red Chamber, a masterpiece exploring the decline of a rich family at the height of the Qing empire in the late 18th century.

But Yu developed the philosophical and historical arguments perhaps implicit in the thought of his mentors: liberal Confucian values, once freed from the imperial ideology of the dynasties, are essential to democracy: The independent spirit of the scholar both models and creates responsible criticism of politics.

[15] He is the inaugural winner of the Tang Prize in Sinology, which recognizes scholars conducting "revolutionary research" and is selected by the Academia Sinica.