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The calculation is made of the Holdridge aridity index to analyze climate tendency and characteristics of drought/wetness changes for the last 50 years by means of the mean air temperature and precipitation at Tengzhou from 1956 to 2005. The results show that under the influence of the obvious increase trend of annual biotemperature and annual potential evapotranspiration，the indistinctive decrease trend of annual precipitation caused the obvious loss trend of annual water budget and the outstanding increase trend of annual aridity index. The aridity index varies periodically, and drought and wetness stages appear alternately. Generally there are three wetness stages and two drought stages. In 1976, the aridity abruptly turned from wet to dry. Climate types changed markedly after the abrupt climate transformation. Through the movingaverage analysis of the aridity index and the standard deviation, it is found that along with the increase of average aridity index，unusual wetness events obviously decreased，but unusual drought events obviously increased. Along with the increase of the standard deviation, the total frequency of unusual drought/wetness events increased obviously.