As of July 2008, the population of China is 1.3 billion, according to About.com's Geography Guide. That phenomenal number is matched by enormous early population figures.
In "Relocation of Civilization Centers in Ancient China: Environmental Factors," by Duan Chang-Qun, Gan Xue-Chun, Jeanny Wang, and Paul K. Chien, population figures are given as:
during the Xia Dynasty;
during the Qin Dynasty; and
in A.D. 2
the Western Han Dynasty.
After that, the population falls, and then, a millennium later, surpasses the 60 million figure under the Southern Song Dynasty, in 1193, when it hits 76.81 million.
The Qin Dynasty was the first time China was unified under centralized government. With the ending of wars, iron implements, farming techniques, and irrigation were developed. By the time of the Han Dynasty, 40 million hectares of land were required to feed the population, which meant losing forests and grasslands to the needs of cultivation. Massive building projects, appropriation by the nobles of peasant land, soil erosion, and movement of the people helped reduce the population so that by A.D. 225, under the Three Kingdoms Period, the population was down about a third, to 40 million.
"Relocation of Civilization Centers in Ancient China: Environmental Factors," by Duan Chang-Qun, Gan Xue-Chun, Jeanny Wang, and Paul K. Chien, from Ambio, Vol. 27, No. 7 (Nov., 1998), pp. 572-575.