Self-thinning is a hot issue in ecological theoretical research and has important practical significance in terms of guiding forestry production. Adoption of dynamic data on the number of trees in a stand and the size of the stand in investigating self-thinning is the key to establishing stand density theory. Taking data from 14 investigations on 14 natural broad-leaved stands in Baishilazi Nature Reserve in Northeast China over the past 30 years, this paper provides evidence for establishing a theory of stand density control. This paper analyzes the variation range for parameter β within the empirical equation N=kD^-β described by Reineke, where N is the stand density and D is the average diameter at breast height (DBH), and explains factors that may make β constant. Results show that with a reduction in stand density, the DBH of a stand increases year by year. Moreover, among the 14 natural broad-leaved communities studied, the power functions of the DBH and stand density both become significant; the exponent β for the No. 17, 33, 35, 38, 49, 50, 61, 67, and 68 natural broadleaved communities is about 1.5, and the variation in β for the No. 1, 9, 22, 23, and 56 natural broadleaved communities is 0.44-1.80. Through further analysis of the fluctuation in β for the five latter stands, it is clear that No. 1, 9, and 56 stands are disturbed by obvious human activities. The number of dead plants in stand No. 22 was 387.5 per hectare in the 15 years from 1993 to 2008 but only 200 in the 12 years from 1979 to 1991, an increase of a factor of 1.94. Therefore, this paper chooses data from 1993 to 2008 for simulation, for which β is 1.57. The average DBH of stand No. 23 increased only 1.95 cm in the 13 years from 1993 to 2008 but 3.72 cm in the 16 years from 1979 to 1993. Thus, this paper adopts data for the 16 years from 1979 to 1993 for simulation, for which β is 1.59. In the self-thinning process, the distribution of tree mortality can be fitted to a unimodal function with a peak skewed to smaller trees, resulting in low mortality for trees with large diameter and high mortality for suppressed trees with small and medium diameters. Meanwhile, a negative feedback relationship forms between the upper- and lower-layer trees. Overall, our results indicate that self-thinning in natural broadleaved forests is a continuous process that is in accordance with a -3/2 power law. The power index β found in this study is constant-with a value of -3/2-and correlates with human disturbance rather than the forest site, species composition, and other factors. Self-thinning in natural forests is the consequence of the negative feedback relationship noted above and is controlled primarily by light intensity. Self-thinning in natural broad-leaved forests is in line with the -3/2 power law, which shows that forest ecosystems of the nature reserve are strongly self-regulating and have a certain stability. Therefore, the avoidance of human disturbance whenever possible, and the maintaining and ensuring of the stability and sustainability of forest ecosystems are necessary in forest ecosystem management.