The optimized allocation of population distribution should not only consider the cost and revenue of Population aggregation, but also should pay attention to the restriction and suitability level of human settlements. First, by building the revenue function and external cost function of population, this paper analyzed the optimal population for the central city, suburb area and county area of Nanjing city. Then by using random and objective function based on evaluation for the suitability level of human settlements, taking the forecasted total population and optimal population for circle areas as constraints, and using Monte Carlo methods, analyzed the population’s optimized allocation distribution. Compared with current population distribution, this paper divided the studied area in to four different types for population growth and distribution. Finally, this study showed that, population densities of business and older areas like Hunan Road, Xinjiekou, Hongwulu, Chaotiangong Streets are so high that there are needs of controlling the population size.The comprehensive suitability levels for population distribution in the suburb area, like Dongshan, Yanjiang, Xianlin towns, are relatively higher. Considering the faster economic growth rate and lower population density of these areas, population aggregation should be encouraged.The ecological sensitivity of northern, southern parts and ecological protecting areas are relatively higher, which means the population growth rate should be controlled, and people in these regions should be guided to move to other areas.