In this study, we analyzed the spatio-temporal pattern changes and the dynamic mechanism of evolution as a warning index of forest ecological security, to provide a theoretical reference for decision-making in forestry ecological security in Beijing. This study focused on the establishment of an index system as a warning for forestry ecological security. By analyzing spatio-temporal differences from 2009 to 2015 through SD models, a prediction of the evolutionary trend for the ecological security of the forest from 2015 to 2030 was made. The results indicate that:(1) Forestry ecological security will improve from 2009 to 2030 overall. The warning index decreased slightly from 2009 to 2015, but will increase from 2016 to 2030 as predicted. (2) The early warning index decreased significantly in all the five counties and districts, with the Haidian District presenting the sharpest drop. From 2016 to 2030, with the exception of Haidian District, a slight increase can be seen, which shows that ecological security has improved. However, early warnings in the Shijingshan District are still at high levels, and the Haidian and Chaoyang Districts have a severe warning. (3) Spatially, the early warning index demonstrates an upward trend from city function extension zones to new urban development zones followed by ecological conservation zones, and the low index range increasingly expands with urbanization. The security conditions in ecological conservation zones are much better than those in other areas. The early warning index has improved in new urban development zones but has declined in some parts as a function of the extension zone. (4) The spatio-temporal evolution features of the early warning index of forest ecological security in Beijing are significantly affected by many factors, such as forestry resources, social economy, natural environment, and forestry policy, which results in the periodicity and the regional differences of the evolution of forest ecological security.