The intricate relationship between prestimulus alpha oscillations and visual contrast detection variability has been the focus of numerous studies. However, the causal impact of prestimulus alpha traveling waves on visual contrast detection remains largely unexplored. In our research, we sought to discern the causal link between prestimulus alpha traveling waves and visual contrast detection across different levels of mental fatigue. Using electroencephalography (EEG) alongside a visual detection task with 30 healthy adults (13 females; 17 males), we identified a robust negative correlation between prestimulus alpha forward traveling waves (FTW) and visual contrast threshold (VCT). Inspired by this correlation, we utilized 45°/-45° phase-shifted transcranial alternating current stimulation (tACS) in a sham-controlled, double-blind, within-subject experiment with 33 healthy adults (23 females; 10 males) to directly modulate these alpha traveling waves. After the application of 45° phase-shifted tACS, we observed a substantial decrease in FTW and an increase in backward traveling waves (BTW), along with a concurrent increase in VCT, compared with the sham condition. These changes were particularly pronounced under low fatigue state. The findings of state-dependent tACS effects reveal the potential causal role of prestimulus alpha traveling waves in visual contrast detection. Moreover, our study highlights the potential of 45°/-45° phase-shifted tACS in cognitive modulation and therapeutic applications.Significance Statement Visual contrast detection, despite consistent stimuli, frequently exhibits variability. This variability has been linked to prestimulus alpha neural oscillations in prior studies. Recently, there has been increased interest in exploring large-scale alpha traveling waves and their connection with visual processing. Yet, the role of these traveling waves in visual contrast detection remains unclarified. Through a combination of visual detection tasks, EEG data analysis, and 45°/-45° phase-shifted tACS, our study elucidates how prestimulus alpha traveling waves exert a potential causal influence on visual contrast detection.