Right after finishing graduate studies in ichthyology in 1950, Li initially worked on Chinese nomenclature of vertebrates, specializing in fishes, within the publication office of the newly-formed Chinese Academy of Sciences. Later in the year, he joined a team of zoologists engaged in consolidating and sorting out zoological specimen inherited from Fan Memorial Institute of Biology and National Academy of Peiping, two predecessors of the Academy of Sciences, in preparation of the later formation of the Institute of Zoology. Starting from the early 1950s, Li helped to initiate and participated in China's systematic surveys of marine fishes in Bohai Sea, Yellow Sea and South China Sea, along China's coast. His scientific career, however, was interrupted in 1957 when he was labelled a "rightist" during the Anti-Rightist Movement. From then until the mid-1970s, while his research and publication capacity had been severely impacted by whims of political campaigns and turmoils, he managed to conduct surveys of freshwater fishes in the Yellow River, Ou River and Ling River, as well as in interior regions such as Xinjiang and Gansu provinces. While blacklisted as a "rightist" and assigned to work on aquaculture-related research in the early 1960s, he discovered a salmonid fish serendipitously in the vicinity of Qinling Mountains, which is later named Brachymystax tsinlingensis Li, 1966. For a relatively tranquil period of about a year before the Cultural Revolution, he was allowed to publish research results related to the salmonid fish, fishes in the Yellow River, and earlier surveys of marine fishes. But only until after the Cultural Revolution, Li was able to resume normal research activities. In his book Studies on zoogeographical divisions for fresh water fishes of China, Li divided the fauna of freshwater fishes in China into five major regions based on characteristics of fish species distributions, geographic environments and geological histories of these regions. According to his method, the demarcation line between Holarctic and Indomalayan realms in China, as far as freshwater fishes are concerned, lies largely along the Himalayas and Nanling Mountains ranges. This contrasts with the prevalent view of demarcation line between Holarctic and Indomalaya regions along the Qinling Mountains, even though the latter does not extend into Eastern part of China. Freshwater Ecoregions of the World, a collaborative global biodiversity project partly sponsored by World Wide Fund for Nature, cites this book as a source of references in delineation of freshwater ecoregions of China. Two of his books are published posthumously. In Fishes of the Yellow River and Beyond, he described over 170 native fish species and the characteristics of fish distribution in the Yellow River and its tributaries; a collection of his papers and popular science articles, as well as a personal memoir, are also included in the posthumous book published in Taiwan; this book was re-published in the mainland in 2017. Comparing fish species in the Yellow River in 2010–2015 and the survey results originally reported by Li in 1965 as part of his research on the book, a recent study by a Chinese team concludes that only about half of the native fishes in the Yellow River could still be found, due to anthropogenic environmental alterations and increased presence of introduced species over the past half-century. Li was also the primary contributor to a volume of the Fauna Sinica series covering the orders of Atheriniformes, Cyprinodontiformes, Beloniformes, Ophidiiformes and Gadiformes, which was published posthumously in 2011. Another volume of the Fauna Sinica series which he worked on during his last years, covering fish species in China within the orders of Beryciformes, Zeiformes, Lampriformes, Gasterosteiformes, Mugiliformes and Synbranchiformes, is yet to be published.
