The arid region in northwest China is one of the most sensitive areas to global change. Global warming accelerates water cycle,causes obviously the change of water cycle in atmosphere. In this paper, the characteristics of water cycle in atmosphere and the related scientific problems in the arid region in northwest China were analyzed based on the data analysis and the newest research results. The daily temperature,precipitation and relative humidity data from 96 meteorological stations in the study area were analyzed by the Mann-Kendall test and linear trend. The analyzed results showed that water vapor and precipitation experienced a sharp increase in the late 1980's,however, such sharp increase trend has been replaced by an obvious decrease since the 21st century. Water vapor and precipitation over the east of northwest China decreased obviously due to the weakening of monsoon intensity. It was sensitive to the change of water vapor in northwest Xinjiang and the northwest of the Hexi Corridor. Proportion of the stations where precipitation increased was as high as 95.9%,especially in the Tianshan Mountains and the Qilian Mountain. Precipitation was increased significantly with the increase of elevation except that in a vertical zone from 500 to 1 500 m a. s. l. Precipitation was increased by 7 mm·(10a) ~(- 1) when the elevation was increased by 1 000 m in a vertical zone from 1 000 to 2 000 m a. s. l.,and by 10 mm·(10a) ~(- 1) in a vertical zone from 2 000 to 4 000 m a. s. l. Average water vapor balance was 2.8 * 10~(11) m~3, the precipitation cycle rate was low,and the net water income in central troposphere was increased significantly. Actual evaporation was slightly decreased in the arid region in northwest China,and its reduction in the Tianshan Mountains was significant,but it was increased significantly in the Qilian Mountain.