Abstract: This paper analyzes Aldo Leopold’s thinking of forest system health in his work of A Sand County Almanac at the beginning of the 20th century, who was a forestry specialist and pioneer of environmental ethic and protection in American. The paper concludes four aspects of his profound thinking: disease trees supply house and food for animals; animals can control tree diseases and insect quantity; there are complicated and subtle relations between animals and plants, and among plants; disease trees improve the aesthetics value and vitality and vigor of forest. Finally, a summary to Leopold’ thinking is made.