In this paper, a new low-pressure die-casting with degassing function, casting under adjusted pressure (CAP), is proposed. The working principle of CAP process is introduced in details. This paper focuses on the influences of moisture of compressed air on the hydrogen content within the melt aluminum during the cyclic vacuum-pressurizing programs for CAP process. The experimental results indicate that degassing under vacuum acts positive in refining aluminum melt. The relative moisture of compressed air exerts a significant influence on the hydrogen content of aluminum melt. After the melt aluminum is exposed to the air with a relative moisture of 85% for 90 min, the hydrogen content of the melt aluminum can get to 0.4 ml/100g/Al; while under the circumstance of CAP process, the hydrogen content is maintained between 0.05 and 0.15 ml/100g/Al depending on the moisture capacity of the air, which virtually has no more influence on the casting qualities. Under the condition of dry compressed air, the hydrogen content can be kept at a level as low as 0.05ml/100g/Al, which facilitates to obtain high air-tightness castings.