This study investigates the role of internal climate variability in the trend reversal of reference crop evapotranspiration (ETo) in the arid region of Xinjiang, Northwest China, and quantifies the relative contributions of internal climate variability and anthropogenic irrigation activities to this trend change. The findings reveal a distinct trend reversal in ETo in Xinjiang around 1998, transitioning from a decreasing to an increasing trend. Internal climate variability and irrigation activities within the region's irrigation stations drove this abrupt shift, alongside the influence of the Pacific Decadal Oscillation (PDO), which emerged as a key contributor to the trend reversal. Specifically, the PDO's phase shift from positive to negative in 1998 triggered a reversal in wind speed (U), changing its trend from decreasing to increasing, while concurrently, fluctuations in the Atlantic Multidecadal Oscillation (AMO) also impacted U, with these collective variations in PDO and AMO indirectly influencing ETo by modulating U. Additionally, irrigation activities contributed to a shift in relative humidity from an upward to a downward trend around 1998. These changes collectively played a critical role in shaping the ETo trend turning point. Quantitative attribution analysis determined that internal climate variability explained 68.26 % of the post-1998 ETo increase and was central to the trend reversal, while irrigation activities contributed 27.18 % to amplifying the shift. These findings underscore the compound effects of internal climate variability and anthropogenic factors on regional hydrology in arid zones, highlighting the intricate interplay between natural climate patterns and human-induced changes.