Abstract: To address the limitation that traditional static assessments mostly rely on one-time measurement on the day of water recession and fail to capture lagged ecological impacts of flood disasters, this study takes the extreme "8·7" flood disaster in Yuzhong, Gansu Province in 2025 as the research object, and constructs a multi-spatiotemporal scales assessment system with three nested scales (landscape, ecosystem, and community) that integrates both ecological structure and function. Based on the Google Earth Engine (GEE) cloud platform, with the ecological status of 3-5 years before the disaster as the baseline, quantitative accounting of ecological damage was carried out at three time points: 0 day, 10 days, and 30 days after water recession. The results showed that: Within 30 days after water recession, macroscopic ecological structure indicators such as vegetation coverage and landscape pattern rebounded rapidly, while community structure and ecological regulation functions including water conservation and carbon sequestration continued to decline, presenting significantly asynchronous change processes. Ecological functional damage exhibited a prominent lag characteristic. The period of 10-30 days after water recession was the concentrated emergence stage of functional damage, with the area of moderate functional damage increasing by 233% compared with that on the day of water recession. A static assessment conducted only on the day of water recession would miss 15%-25% of latent functional losses. This flood disaster has dual effects of disturbance and replenishment on the regional ecosystem. A total of 66.30% of the non-inundated areas across the study region showed significant ecological improvement due to precipitation replenishment. This assessment method compensates for the shortcomings of traditional static ecological damage assessment, and can provide technical support and decision-making basis for accurate flood damage assessment and post-disaster ecological restoration in the ecologically fragile semi-arid regions of Northwest China.