Populus euphratica is a member of the Salicaceae family, and is a key tree species of the desert riparian forest. Forest gaps can promote the natural renewal and succession of forests and optimize forest structures and nutrient cycling, thereby, which is regarded as a main driver for increasing biodiversity and ecosystem services. In this study, field data were obtained from six plots with a typical sampling method within study area. The effects of hydrological conditions on the number of canopy gaps, formation and dynamics of gap maker populations were analyzed by using a combined method of the regression model, the goodness-of-fit t test and the dynamic quantitative of population structure, based on the hydrological data such as runoff, water consumption and groundwater depth in the middle reaches of the Tarim River. The result showed that:1) Before the 1970s, the amount of water in the middle reaches of the Tarim River was sufficient, to a certain extent, which could meet the water requirement for the survival and development of P. euphratica forests to some extent. Since the beginning of 1984, the annual runoff (R²=0.785, P < 0.01) and water consumption (R²=0.5238, P < 0.01) in the middle reaches of the Tarim River have shown a significant decreasing trend, the groundwater is increasingly scarce and its depth is in a declining trend (R²=0.8618, P < 0.01), especially during the period of 1997-2006. 2) As the annual runoff and water consumption gradually decrease, the groundwater depth also increases; the number of gap maker population becomes a growing trend. 3) In addition to the recession type in 1957-1966 and 1967-1976, the forest gap maker population showed a growing trend in 1977-1986, 1987-1996, 1997-2006 and 2007-2016. Particularly, the juvenile P. euphratica tree gap makers in the forest gap increased significantly, which showed the aging characteristics of P. euphratica forests. 4) According to the model output, there is a significant negative correlation between runoff and the number of canopy gaps, and gap makers. Large number of P. euphratica in forest gaps will face with mortality, when runoff is decreased to the 17.81×10⁸ m³. Thus, hydrological conditions in the arid area are the dominant factors determining the life and death of P. euphratica, and also the main driver for the formation of desert riparian forest gaps in the middle reaches of the Tarim River.