Following the end of the Chinese Civil War in 1949, Li moved to Beijing, where he served the Communist government as an Expert Commissioner to the National Law Commission of China up to 1956. In that year, he was made Professor of International Law at the College of Foreign Relations. From 1963 to 1993, Li was concurrently Professor of International Law at Peking University and Legal Advisor to the Ministry of Foreign Affairs of the People's Republic of China (PRC). In 1979, he completed drafting the first Criminal Law of the People's Republic of China and the Law of Criminal Procedure, a key piece of legislation enacted as part of a return to legality under the Open Door Policy.