Abstract: It is well known that the moist available energy (MAE) plays an important role on the broken, development and maintenance of the heavy rain. In order to study the MAE of the Huanan heavy rain and the relationship of the Huanan heavy rain to the MAE, the MAE of three heavy rain fall cases in Guangdong Province in 1998 are calculated using developed formula of MAE, one case is on 14—15 May, the other two cases are on 23—24 May and on 8—9 June. The evolvement and convergence of the heavy rain MAE are analyzed. The space-time relationship between the MAE and the heavy rain is also studied. It shows that the MAE at Guangdong Province and the surrounding area plays an important role on the heavy rain fall period. The high MAE at Guangdong Province supplies the energy for the heavy rain. The low MAE over the middle reaches of Changjiang River helps to form the energy front, at the same time, it can result in heavy rain fall directly or indirectly.The MAE of the heavy rain has quite different in the early and later period of the heavy rain. The evolution of the MAE during the heavy rain procedure oscillates obviously. The MAE is high in the pre-heavy rain period but low after the rain. The difference value between the early and later rain fall period exceeds 6×10⁵ J/m². In order to study the energy threshold of the heavy rainfall, the MAE relative value is defined. This item increases obviously before the heavy rainfall period in the three cases. It indicates that the MAE relative value can reveal the MAE variety characteristic before the heavy rainfall period. As a result, this relative value can be used as a criterion in forecasting the heavy rainfall in the future. The strong rainfall period in the three heavy rain cases is in good agreement with the highest difference value of the MAE between the north and south area. The difference value of the MAE between the north and south area increases because the low MAE value area in the middle reaches of Changjiang River strengthens and moves south wards, which can break the front heavy rain. It means that the low MAE over the middle reaches of Changjiang River plays an important role on the three cases of the heavy rain, it also means that the mid-latitudes is the key regions for the Guangdong heavy rain. The study on MAE convergences of the three heavy rains also shows that the MAE converges at the low and middle troposphere mostly. The MAE convergence at the low and middle troposphere is the main accumulation and supplement of the MAE. It suggests that the strong rainfall period is in good agreement with highest value period of the MAE, at the same time the MAE convergence amount in Guangdong Province is much higher than that over the middle reaches of Changjiang River. In conclusion, the MAE plays an important role on the heavy rain.