We calculated and analyzed the dynamic variation of aridity index in China during the 20th century. The results were interpolated by using GIS software,the digital maps of aridity index distribution were made,and the drought trend was discussed. The results show that humid climate area in China reduced gradually, semi-humid and semi-arid climate areas increased especially,and the arid area increased too. From the southeast to the northwest in China,the climate was gradually drying up in 20th century. The arid area in the northwest and north of China expanded to the south and east.