Ecological security is the state when an ecosystem maintains its stability under external stress. Due to climate change and the increase in human activities since the 20th century, the rapid decline in global ecological security has threatened sustainable human development. The evaluation and projection of global ecological security is important for forming adaptation strategies to maintain sustainable development in sensitive areas. However, the current assessments of ecological security mainly focus on regional scales, and the interactions among different factors have not been considered, resulting in future projections having substantial uncertainty. Here, a new index of ecological security was developed by including biological, oxygen, carbon, thermal and hydrological cycles and the impacts to ecosystem stability from climate change and human activities at a global scale. A global distribution map of ecological security has been established that covers the past 60 years and includes projections for the future 100 years. A severe decline in ecological security has occurred in drylands that has expanded into surrounding regions over the past 60 years. The response of ecological security to global warming and human activities is projected to be stronger. By ~2100, under a high greenhouse gas emissions scenario, the amount of globally insecure land would cover more than 57% of the land in the world.