With the aid of the atmospheric entropy balance equation, the diagnostic and comparative analyses of the entropy reduction motion under the action of the negative entropy flow during the typhoon heavy rain have been conducted. The results are as follows: (1) The large-scale heavy rain caused by typhoon corresponds with intense and negative entropy change. So it is better than the conventional synoptic method to resolve the heavy rain areas. (2) The interaction between synoptic systems of the mid and low-latitude belts can be clearly represented in the confluence of negative entropy changes which provide a sign of the intense and hard rain processes or hardstorm rainfall affected by the typhoon far from rain area. (3) It can be explained by the equation of entropy density variation that the genesis and development of synoptic system in the rain area are associated with the physical process of the atmospheric entropy reduction resulted from precipitation. (4) The effect of atmospheric entropy divergence, especially in mid- and upper-levels, on the occurrence and maintenance of heavy rain is as important as heating, even more important.