Mental fatigue is a serious problem in contemporary society, which affects personal productivity, safety in life and mental health, and it has become a convergent research topic in the nascent field of neuro-ergonomics. However, we still lack a clear understanding of the neural mechanism of mental fatigue, which contradicts the realistic demands. In this study, considering the dynamic accumulation of mental fatigue, temporal brain networks were estimated from 20 participants based on functional images. First, the dynamic connection network was constructed through functional imaging data in a sustained visual attention task of 32.6 minutes. Quantitative analysis of the dynamic functional connectivity (FC) was then performed in the 3D spatiotemporal architecture using our recently introduced temporal efficiency method. We found that the temporal global efficiency was decreased with the increase of mental fatigue, suggesting a significantly disintegrated spatiotemporal topology of dynamic FC underpinning mental fatigue. The finding therefore expands the research of related static brain networks and provides new evidence for the resource hypothesis of mental fatigue.