Abstract: Fragile ecological environment and shortage of water resources in Northwest China have seriously affected agricultural and economic development. Among them, the precipitation form can be one of the most important factors in the water cycle in the northwest arid region. The changes in precipitation forms can be of great significance to determining the water resources change in the background of climate warming-wetting trend in northwest China. In this study, the wet bulb temperature dynamic threshold model was established to separate the snowfall and rainfall using the daily precipitation, air temperature, relative humidity, atmospheric pressure, and station altitude data observed by the National Meteorological Station in northwest China during 1961-2017. The accuracy of the separation was also validated. A systematic analysis was made on the temporal and spatial variation characteristics of precipitation, rainfall, snowfall, and S/R in the region. Furthermore, the regional differences of precipitation in different forms and the S/R ratio were determined on the spatial scale of the secondary basin. The snowfall was divided into four types, in order to analyze the change characteristics of different types of snowfall. An investigation was implemented to explore the changes in snowfall and rainfall separation threshold and the influencing factors of precipitation forms. The results show that the precipitation in different forms in Northwest China showed a significant increasing trend during 1961-2017. However, the increasing trend of rainfall was more outstanding than that of snowfall. The Snowfall/Rainfall ratio showed a non-significant decreasing trend, with a decrease of 0.07%/10a. The spatial variation trend shared the outstanding regional differences with an "increase in the west and decrease in the east", and the distribution of regions with the outstanding increase was concentrated in the central region of the northern Xinjiang and Hexi Corridor. The Snowfall/Rainfall ratio is distributed with "an increase in the west and decrease in the east", and "increase in the north and decrease in the south". The significant increase of extreme snowfall in Northwest China led to a significant increase of snowfall, with an increase of 1.07 mm/10a. Specifically, the increased rate of extreme snowfall was 1.03 mm/10 a. On the contrary, there was a more outstandingly decreasing trend in light snowfall and moderate snowfall. The slight decreasing trend of snowfall days was mainly caused by the significant reduction of light snowfall days and the significant increase of extreme snowfall days. By contrast, there was no change in moderate snowfall days and heavy snowfall days. The threshold temperature of snowfall and rainfall separation fluctuated greatly on a seasonal scale, with the highest threshold temperature in autumn, the second in winter, and the lowest in spring. The change in relative humidity caused the change in threshold temperature. Furthermore, the higher the relative humidity was, the greater the threshold temperature difference was. On the interannual scale, the threshold temperature showed a non-significant decrease trend, with a mutation change in 2008. On the watershed scale, the change rate of precipitation in each phase increased with the increase of altitude, and the Snowfall/Rainfall ratio was the opposite, the change of snowfall was mainly affected by temperature and relative humidity, and the change of Snowfall/Rainfall ratio was mainly affected by temperature in Northwest China. This finding can greatly contribute to the scientific understanding of the changing of water resources in Northwest China. It is of great significance to the rational regulation of regional hydrology and water resources.