Hydrogen is the only gas that is appreciably soluble in aluminum and its alloy. Its solubility is small but the effect is significant on the mechanical properties and finishing characteristics of aluminum alloys. In order to remove hydrogen from the aluminum melt and shape intricate castings, a new process, Casting under Adjusted Pressure (CAP for short), was invented. An interesting hydrogen fluctuation phenomenon has been found in the CAP process. The hydrogen concentration within the molten aluminum changes with cyclic vacuuming-pressurizing programs for CAP process. The experimental results indicate that the relative humidity of compressed air exerts a significant influence on the hydrogen concentration of aluminum melt. The hydrogen concentration is maintained between 0.10 and 0.15 ml/100gAl by using compressed air with relative humidity 80-85%. Under the condition of dry compressed air, the hydrogen concentration can be maintained at a level as low as 0.05-0.08 ml/100gAl, which is beneficial to obtain high air-tightness aluminum alloy castings. In addition, the experiment shows that magnesium concentration has no obvious influences on CAP process.