Before Qin and Han Dynasties, people used the diameters of one week and three weeks as pi, which was the ratio in ancient times. Later, it was found that the error of the ancient ratio was too large, and the pi should be a diameter of one week, which is greater than the diameter of three weeks, but there are different opinions on how much is left. Until the Three Kingdoms period, Liu Hui put forward a scientific method to calculate pi-secant method, which approximated the circumference of a circle with the circumference inscribed by a regular polygon. Liu Hui calculated that the circle is inscribed with 96 polygons, and π = 3. The more sides inscribed in a regular polygon, the more accurate the π value will be. On the basis of predecessors' achievements, Zu Chongzhi worked hard and calculated repeatedly, and found that π was between 3. 14 15926 and 3. 14 15927. He also got an approximation in the form of π fraction as the reduction rate and density rate.

Liu Hui (about AD 225 -295), a native of zouping county, Binzhou, Shandong Province, was a great mathematician in Wei and Jin Dynasties and one of the founders of China's classical mathematical theory. He is a very great mathematician in the history of Chinese mathematics. His representative works "Nine Arithmetic Notes" and "Arithmetic on the Island" are China's most precious mathematical heritage. Liu Hui has quick thinking and flexible methods. He advocates reasoning and intuition. He was the first person in China who explicitly advocated using logical reasoning to demonstrate mathematical propositions.

Zu Chongzhi (AD 429-500) was born in Laiyuan County, Hebei Province during the Northern and Southern Dynasties. He read many books on astronomy and mathematics since childhood, studied hard and practiced hard, and finally made him an outstanding mathematician and astronomer in ancient China.

Zu Chongzhi's outstanding achievement in mathematics is about the calculation of pi.