A crucial issue in evolution of severe storm, such as typhoon and even single supercell, is to diagnose and predict the sudden intensifying in storm. This paper describes an attempt to investigate the influence and effect of ambient moisture in the development of storm. It was mainly through a dynamic way to detect the helical enhancement by ambient moisture. It was found that the correlation between the ambient moistness and the intensity of rotating convective cells can be well analyzed by helicity dynamics. The correlation between environmental streamwise vorticity (i.e. helicity) and vertical velocity (storm updraft or downdraft) is a powerful indicator to catch favorable or unfavorable conditions for storm evolution. For this purpose, a three-dimensional non-hydrostatic storm-scale model, Advanced Regional Prediction System (ARPS) was employed in order to numerically simulate a well-documented case of Del City supercell storm in different kinds of environmental moistness. Moreover, such different kinds of ambient moist environment and the resultant different morphologies and evolutions in the storm clearly demonstrated that the abundant moistness in the environment that the storm embedded significantly strengthens the storm intensity. In fact, the storm is embedded in much moist environment and therefore larger instability propagates faster than the one in the drier air. It further produces stronger low-level mesocyclone with a much longer lifetime. The stronger convection and twist in the updraft indicates that helical enhancement effect by moisture lead to stronger tornadic activity in severe storm.