Dew is an important water source for natural organisms in arid and semi-arid areas, playing a crucial role in maintaining the stability and sustainability of desert ecosystems. Effectively estimating dew quantity and its long-term changes remains a challenge. Based on conventional meteorological observation data, this study used a Random Forest model to estimate the dew quantity in Nilka, the Southern slope of Boluohuoluo Mountain in middle Tianshan Mountains from June to October in 1970-2022 and analyzed its long-term variations using a statistical method. The results revealed that (1) monthly dewfall varied from 0.74 to 3.88 mm. The 53-year average of the total dew amount in October was significantly higher than in other months (2.81 mm), while the lowest was in August (2.02 mm). In addition, the total dew amount in June, July, and September were 2.27 mm, 2.19 mm, and 2.16 mm, respectively. (2) From 1970 to the beginning of the 21st century, there was a slight decrease in dew from June to October and in every month individually, followed by an increase for about 18 years, after which the dew amount decreased again. During 1970 to 2022, the dew amount exhibited a declining trend when considering the June-October period as a whole or for August and October individually. (3) The change in dew amount was primarily affected by the relative humidity. The findings have implications for assessing the effect of climate change on the dew formation, and could be conducive to further maintaining ecological stability and sustainability in dryland regions amidst global warming.