An automatic injection device for fluid has a sleeve, an actuating unit, a barrel with a piercing needle and a high-pressure air source. The high-pressure air source is mounted slidably in the barrel. The actuating unit is mounted in the sleeve and barrel and selectively blocks the high-pressure air source. When the user needs to release the high-pressure air in the high-pressure air source, the user press the actuating unit to allow 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.