An automatic injection device for fluid has a sleeve, an actuating unit, a barrel with a piercing needle, a high-pressure air source and a driven unit. The actuating unit is mounted in the sleeve. The high-pressure air source and the driven unit are mounted slidably in the barrel. The barrel, the high-pressure air source and the driven unit are first mounted in the sleeve to be ready for use. When the user needs to release the high-pressure air in the high-pressure air source, the user press the actuating unit to drive the driven unit to slide. Then the driven unit also drives the high-pressure air source to slide until the high-pressure air source hits the piercing needle. Therefore, the high-pressure air in the high-pressure air source is easily released by actuate the actuating unit without additional hand tools.
