Abstract: A low-pass filter is used to separte the flow into a larger and a subsynoptic-cale component. Kinetic energy (KE)exchange between these two kinds of motions in the heavy precipitation systems are evaluated. The results show that the subsynoptic-scale motions play a significant role in KE balance of the flow. Subsynoptic- scale motion converter KE into synoptic-scale and acts as a "KE source" of the synoptic-scale. The maximum conversion occurs in the upper troposphere (300-100 mb).While the opposite conversion takes place below 500 mb. This study also indicates that the application of smoothing in the numerical forecast should be careful because excessive smooth will filtrate out the subsynoptic-scale motions which are very important in KE balance. The technique present in this paper can also been used to evaluate the interactions between the motions of various scales.