Abstract: Dew is one of the most important supplementary water sources in arid and semi-arid regions. The influence of dew amount on the water balance can be evaluated for water management, ecosystem protection, and the optimization of agricultural practices. Moreover, the influencing factors of dew formation can provide some insights into how climate change may impact water availability and ecosystems. However, it is still unclear in the dew formation, due to the difficulty of observation without considering the dew in the water balance. This study aims to quantify the dew amount for a better understanding of dew formation. A series of lysimeter experiments were carried out to continuously observe the dew amount in the Mu Us desert from October 2020 to September 2021. Among them, the dew amount included the fog, dew, and hoar frost formation. In addition, the meteorological factors were monitored at the experimental site, including air temperature, precipitation, wind speed, relative humidity, net radiation, and surface temperature. All data were recorded every ten minutes. The dew amount was evaluated for the water balance during formation. Boosted regression tree (BRT) model was used to investigate the influence of meteorological factors on dew formation. The results were as follows. 1) The dew amount was 47.1 mm in the experimental period, corresponding to 19.6% of the annual rainfall. The dew started to form at night when the air temperature was significantly higher than the soil surface temperature. The dew was mainly formed from October 2020 to January 2021 (cold months), with a maximum amount of 9.00 mm in December 2020, and a minimum of 0.55 mm in June 2021. The dew amount was larger in the colder months than that in the warmer ones. 2) The BRT model showed that the largest contribution (43.4%) was found in the relative humidity to the dew formation. It infers that relative humidity was a key factor in the formation of dew. The contribution rates of the net radiation at night and the wind speed to the dew formation were 21.7% and 15.2%, respectively. The much fewer contributions to the dew formation were the air temperature and vapor pressure deficit. The optimal combination of parameters was achieved in the dew formation, such as the relative humidity greater than 53%, net radiation at night less than -37 W/m2, and wind speeds ranging from 0.5 to 1.8 m/s. 3) The surrounding air temperature and relative humidity of Artemisia ordosica posed the more significant influence on the dew formation, compared with bare ground. The errors of the water balance were reduced from 25.3% to 5.7%, considering the amount of dew in the semi-arid regions. The dew amount was quantified to improve the accuracy of the water balance. The finding can provide a strong reference to accurately evaluate the water resources and then better understand the formation process of dew amounts under the coverage of Artemisia ordosica. In addition, the Artemisia ordosica in the water cycle can be expected to protect the vegetation ecosystems in the arid and semi-arid regions.