Based on big-data analysis, this paper used the data of the sixth census in 2010 to calculate the average population density of each county. Based on Krugman's geographic nature features, including synthesis of altitude, environmental vulnerability, and productive potentiality of agriculture, human climate index, and appropriate deviation degree of water resources, traffic convenience, the locational index and GDP were selected, the geographical and ecological basis of HU Line existence are analyzed. The results showed that:(1) China's population density was closely related to ecological factors such as synthetic altitude, environmental vulnerability, human climate index, appropriate deviation degree of water resources, and productive potentiality of agriculture, and was significantly related to traffic convenience, GDP, and location index. (2) In most provinces, the synthetic altitude, productive potentiality of agriculture and appropriate deviation degree of water resources contributed most to the population density, followed by transportation convenience and location index. They wew the first nature and the second nature factors recognized by Krugman's new geography. (3) The population distribution is special in Xinjiang, Shandong, Ningxia, Inner Mongolia, Heilongjiang, Jiangsu, Beijing and Tianjin, and new factors should be introduced to explain the population distribution. (4) The HU Line is an ecologically fragile zone. This paper presents the geographical and ecological basis of HU Line, which is the dividing line of spatial distribution of Chinese population.