Two years after the founding of the People's Republic of China, Xia and her husband Yang Liming returned to China in 1951. She took up an appointment as an associate researcher in the Department of Electrical Engineering at Tsinghua University.
In the fall of 1952, mathematician Hua Luogeng initiated the development of China's first electronic computer, and recruited Xia and two other scientists to lead the project at the Chinese Academy of Sciences (CAS). This became the turning point in her career. Four years later, she became a founding professor of the Institute of Computing Technology of the CAS, where she spent the remainder of her career. After the two other scientists, Min Naida and Wang Chuanying, both left the project, Xia led the development of China's first indigenously designed general-purpose electronic computer, Model 107, in 1958. She has been acclaimed as the "Mother of Computer Science in China".
Xia made numerous contributions to the research and design of high-speed computers in China. She is also credited with designing a high-speed array processor and a range of multiple parallel computers. She helped establish the Chinese Journal of Computers in 1978 and the Journal of Computer Science and Technology, the only English language journal in the computer field published in China, in 1986.
In March 1956, Xia taught China's first course in computer theory, and wrote Principles of the Electronic Computer, the first systematic computer science textbook in China. When the University of Science and Technology of China was founded in 1958, Xia was in charge of establishing its computer science department. She taught more than 700 students from 1956 to 1962.
Xia advised more than 60 graduate students, two of whom won top national prizes for Ph.D. dissertations. Her students included Li Guojie , who led the development of the Sugon supercomputers; and Hu Weiwu , the chief architect of the Loongson CPU. When Loongson unveiled China's first indigenously designed CPU in 2002, Hu named it Xia-50 to celebrate her five-decade-long career in computer science.
In 1991, Xia Peisu and her husband Yang Liming were both elected as academicians of the Chinese Academy of Sciences. In recognition of her contributions to China's computer industry, the China Computer Federation honoured Xia with its inaugural Lifetime Achievement Award in 2010, along with Zhang Xiaoxiang