Nano-metal nickel powder is prepared by liberty arc in which fusible hydrogen is joined as cathode shoot. It is found that the influence of working pressure do not followed the classical rules. Hydrogen is a key factor to improve the productive efficiency of nano metal powder. The mini-hydrogen bubble in fusible metal can lead to the vacuum evaporating of Nickel, so the powder productivity is proportional to the hydrogen pressure. The molecular evaporating mechanism of hydrogen in Ni bubble is presented.