Li returned to China at the age of 30 and became the Professor of Genetics and Biometry at University of Nanking, his alma mater, in 1943.
After World War II, he moved to Beijing for a Professorship and dean of Agronomy at Peking University in 1946, where he finished An Introduction to Population Genetics in 1948.
[1] Li became persona non grata for studying and teaching genetics following the 1949 establishment of a communist government in mainland China.
[1] In 1950, Li fled with his family to Hong Kong, where he was trapped without documentation of citizenship and unable to obtain a visa.
Friends and colleagues, particularly Nobel laureate H. J. Muller and former US surgeon general Thomas Parran, assisted Li in emigrating to the United States.