There took place an inter-decadal climate change over the west part of Northwestern China at mid-1980s. One of the significant characteristics is the increase in precipitation with more extreme rainfall or snowfall event. For understanding its mechanism our calculation focuses on the difference of the geo-potential height on 500 hPa, transient eddy activity, column water vapor content or precipitable water, moisture transport and its divergence between 1987-2000 and 1973-1986. The results show that the change in height field is favorable for the increase in moisture transport to the region with moisture convergence, leading to more water vapor over the region. Meanwhile the eddy activity became weak in wintertime and strong in summertime, leading to more synoptic processes in summertime and less in wintertime. A further decomposition shows that the increase in the transport with convergence was contributed mainly by its stationary-wave part along the west-east direction, while the transient eddy played a positive role in summertime and a negative one in wintertime for precipitation. It turns out that the increase in precipitation in the region resulted from the moisture convergence and the increase in eddy activity in summertime while the moisture convergence played a dominant role in wintertime. Besides, the warming climate also played a positive role in increasing air water-vapor content in the region since the warm air can hold more water vapors than the cold one.