Abstract: A mathematic model of the growth restricted by the population density of Milu population in Dafeng National Reserve Region in this paper and derived the equilibrium values of the population under different intensity of damage. In the meantime, the critical value of the intensity of damage for keeping the population to survive sustainable was presented. It draws the portrait, in the direction field, of solutions (Milu populations) of the model under different intensity of damage by MATLAB software. It was showed that the greater the intensity of damage is, the lower the equilibrium value is. When the intensity of damage was less than the critical value, the equilibrium value would become greater than the basic population, and the Milu population would survive sustainable. However, the equilibrium value will less than the basic population when the intensity of damage was greater than the critical value. This means that the sustainability of Milus survival will be threatened.