Research on agricultural water consumption in northwest China is crucial to relieve problems associated with the water crisis. Apples are a typical crop in China that have been used in agriculture for centuries. As production increases for economic growth, it is essential to analyze water transport mechanisms and water accessibility to mature apple orchards in arid regions. Trunk diameter fluctuation is an effective indicator of the water condition of plants. Its role as an indicator of plant water conditions thus emphasizes that it is crucial to design an irrigation schedule. The changes in the trunk diameter of mature apple trees (Malus domestica Borkh. cv Golden Delicious) was monitored using dendrometers at the Shiyanghe Experimental Station for Water-saving in Agriculture and Ecology of China Agricultural University (37°52'N, 102°51'E, altitude 1581 m). Stem water potential was measured using a pressure chamber. Wind speed, net radiation, relative humidity, and air temperature were monitored by an automatic meteorological station, and soil moisture was measured every 5 days using Time-Domain Reflectometry (TDR) methods based on the previously described data analysis. Change in the maximum daily trunk diameter (MXTD) and maximum daily diameter shrinkage (MDS), as well as their response to environmental factors were also monitored. The results showed that MXTD occurred at 7:00 to 8:00 in the morning and minimum of daily trunk diameter (MNTD) occurred at approximately 16:00 during sunny or cloudy days in 2008 on a daily scale. The trunk diameter increased at night and decreased during the day, which was meanly depended on reference evapotranspiration. MXTD increased during bud development, flowering, and leaf expansion periods (stage I) and remained constant during the fruit expanding and maturing periods (stage II), which was closely related to the MDS of the apple tree. Higher fruit yields were associated with smaller fruit stem diameter growth. MDS increased at the beginning and then gradually decreased during the entire growing stage. The relationship between MDS and stem water potential was linear and the determination coefficient was 0.76^{* * *}. Consequently, MDS indicated the water status of the mature apple trees. However, MDS was more responsive to net radiation, reference evapotranspiration, vapor pressure deficit, and air temperature at stage II than at stage I because the canopy structure was not developed, and the water stored in the apple trees less frequently fluctuated during stage I. The order of determination coefficient over the whole growing stage was as follows: maximum vapor pressure deficit > maximum air temperature > net radiation. Multiple regression relationships among MDS and environmental factors can be used to calculate the MDS for well water supplied to apple trees, which could be considered as a reference value when the tree requires irrigation. Thus, the fluctuation regularities of tree trunk diameter could reflect the water status of the entire fruit period of fruit trees situated in arid areas of Northwest China and help improve orchard water management, as well as ensure the normal growth of fruit trees.