Using Northwest China as the research area, the bias correction and spatial disaggregation (BCSD) method was used to downscale the five GCMs data of CMIP6. Based on the Penman Monteith formula considering the impact of CO₂ concentration and the effective precipitation calculation method, combined with the single crop coefficient method and the saxton equation, the changes in ecological water demand and ecological water deficit in Northwest China were estimated under moderate forcing (SSP2-4.5) and high forcing (SSP5-8.5) climate scenarios. The results show that the future ecological water demand in Northwest China is increasing at 3.83×10⁸m³/a and 6.95×10⁸m³/a under the SSP2-4.5 and SSP5-8.5 scenarios, respectively. In the late 21st century (from 2070 to 2100), under the SSP2-4.5 and SSP5-8.5 scenarios, the average annual ecological water demand will be 5.679×10¹¹m³ and 5.881×10¹¹m³, respectively, the average annual effective precipitation will be 3.985×10¹¹m³ and 4.106×10¹¹m³, respectively, and the average annual ecological water shortage will be 1.732×10¹¹m³ and 1.798×10¹¹m³, respectively. Northwest China is in a state of water shortage as a whole, with 60.2% of the grid points lacking water, which is mainly distributed in the north and west of Xinjiang, the middle of Qinghai, and most regions of Shaanxi, Gansu, and Ningxia.