The study on regional population evolution is helpful to reveal the rule of population distribution and is important for making reasonable population development policy, urban planning and the regional sustainable development. Taking Jilin Province, one of the old industrial bases in China, as a case, this paper describes the evolving characteristics and forming mechanism of the population density spatial pattern on county level from 1990 to 2010.Based on population census data, it pinpoints the evolution characteristics and spatial pattern of population density in Jilin Province through the spatial autocorrelation analysis, index of population concentration, and trend analysis. It shows that the population density of Jilin Province has "middlehigh and wings-low" feature, and the population growth of Jilin Province is always in centralized and longitudinal pattern. Meanwhile the differences of the spatial pattern of population density are huge, 57.2% districts' spatial patterns of population density has changed over the last 20 years. The trend changes from that assembling on the "single-core", viz. Changchun city, to that assembling on "multi-cores", viz. the prefecture-level cities. Another worthy characteristics that the ability of gathering people of west area becomes stronger than that of the east since the year 2000. Natural environment and historical foundations direct influencing factors that accelerate the spatial unbalanced distribution of population. Flat regions with less disasters and comfortable climate are always the first option to be settled. Economic development, policies and city reconstruction are also play important roles in centralizing people to well-developed districts. Economy is the engine of the population growth and decisive factor that influencing the population distribution, policies and city reconstruction are the crucial factors that influencing the advancement of cities' function and effecting the population distribution indirectly. In the meantime, all the factors interrelate and interact with each other.