Assessment of the suitable scale of oases is an effective way to efficiently and rationally allocate water resources in arid areas. In this study, a suitable oasis scale computation model was adopted based on water-heat balance theory to assess the changes in the scale of oases within the middle reaches of the Heihe River Basin (HRB) located in northwestern China. The results show that the runoff of the HRB was characterized by obviously plentiful and withered periods from 1954 to 2015. Under the premise of ensuring the downstream ecological water utilization target, the suitable scale of oases was 3222-4832 km2, 2583-3875 km² and 2221-3332 km² in high-flow, normal-flow and low-flow years, respectively. The status quo of 2015 far exceeded the suitable scale of oases that could be supported by the water resources in the basin in normal-flow and low-flow years, and the suitability index of the oasis scale declined to 0.44-0.59. To ensure the stability of the oasis system, controlling the scale of oases as much as possible within the range of areas that can be supported by water resources is necessary, as is analyzing in depth the suitable proportions of arable land, grassland, and woodland in the future to establish a multi-benefit oasis ecology protection system, thereby providing guidance for the sustainable development of oases in arid areas.