The present invention relates to 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.本發明為一種流體自動注射裝置，其具有一套筒、一致動單元、一具有穿刺針的筒體、一高壓氣源及一從動單元，制動單元設置於套筒中，高壓氣源及從動單元可滑動地設置於筒體中，筒體、高壓氣源及從動單元先安裝於套筒中以備使用，當使用者需要釋放高壓氣源中的高壓氣體時，使用者按壓致動單元以驅動從動單元滑動，則從動單元亦驅動高壓氣壓源滑動，直到高壓氣源撞擊到穿刺針，因此，高壓氣源中的高壓氣體可藉由致動該制動單元而輕易的釋放，無須使用其餘手工具。
