Abstract: Proceeding from the isograms of longterm mean annual precipitation, runoff and evaporation, this paper introduces chiefly the regional distribution of the three key elements in water balance and their variations through-out the year or from year to year as well as further analyses the related characteristics between the precipitation and the runoff or evaporation in Yangtzer River basin, Yellow River basin and Pearl River basin. Some abnormalities in the relationships, such as the relation between drought index and runoff coefficient, show that pseudo-correlationand and its effects are existed.The runoff probabilities calculated for the three regions reflect better corresponding relationship between the key elements. Based on the data of 13 groups of Yangtze Basin, the author has calculated the evaporations. The examinations can satisfyingly be use in practice in plain, hilly and mountaineous area.