In the extremely arid area with little rainfall, a closed plastic canopy was constructed in the gobi land on the top of Mogao Grottoes in Dunhuang, northwestern China. An air-conditioner was put into the canopy to restrain the “greenhouse effect”. The refrigeration function was used to restrain the raising temperature effect and reduce the humidity. In this way, by making the temperature and humidity inside the canopy the same as those outside, we could evaluate the evaporation intensity of phreatic water in the field by measuring the weight of air-conditioning refrigeration water. The temperature, relative humidity, and absolute humidity at five different heights (+50, -10, -20, -30, -40cm) inside the canopy are higher than those outside. At the corresponding heights, differences of temperature are 3.85, 1.90, 1.18, 0.70 and 051℃, for relative humidity they are 11.3%, 1.00%, 1.03%, 2.07% and 6.45% and for absolute humility they are 384, 1.05, 1.15, 1.22, 1.92g/m3. Even values of the key parameter inside the canopy are higher than those outside, soil temperature and humidity profiles showed little difference, and soil conditions were suitable to the upward migration of water both inside and outside the canopy, so we can eliminate the possibility that the evaporation inside was caused by the “greenhouse effect”. During the 45-day-long observation period, 29572g refrigeration water was collected in total, and the average daily evaporation quantity in the later 20 days was 703.6g and higher than that of 612.7g in the former 25 days. The result is 8 times larger than the data gained in 2008 under a similar condition but without air-conditioner, still it is expected smaller than real values since the inside temperature and humidity were higher than outside due to the low-capacity of the air-conditioner. This demonstrates that the soil water content inside the canopy did not decline but increase slightly even evaporated constantly, and the evaporated and discharged water from soil was not resident rainfall water; phreatic water evaporation is an universal phenomenon in arid areas, and the evaporative capacity is no less than 0.0219 mm/d in this field experiment in which the ground water table is deeper than 200m. Solar radiation intensity, temperature, and relative humidity have close relations with phreatic water evaporation, among which relative humidity shows the highest correlation with the values of evaporation. The corresponding correlation coefficient are 0.474 (P=0.01), 0.376 (P=005) and -0.610 (P=0.01), respectively. Apart from climate factors, soil characteristics of the unsaturated zone including water content, void ratio and salinity also contribute a lot. Preliminary measurement result shows the evaporation of phreatic water, which is the primary water for plants in extremely arid area, is considerable and has great ecological and utility value. This work could be a basis for the assessment of ecological reconstruction with phreatic water in desert area, and is significant to the research of deep buried Groundwater-Soil-Plant-Atmosphere Continuum (GSPAC) hydrology system as well as the exploitation of deep ground water. Since evaporation intensity of phreatic water can be used to ascertain the vapor flux in the adjacent rock around Caverns, this work is also important to the protection of Mogao Grottoes.