The symptoms of brain fatigue are reflected in a decline in cognitive function, which is mainly regulated by the prefrontal cortex (PFC). Functional near-infrared spectroscopy(fNIRS) can measure the changes in cerebral blood oxygen concentration over a period of time, thereby indirectly reflecting the degree of activation of the cerebral cortex. Therefore, a wearable fNIRS headband with 10 channels that can cover the prefrontal area is designed, and the headband is used to detect the subject's brain to observe whether the subject enters the fatigue state.The headband uses 780 nm and 850 nm laser diodes and silicon photodiodes as light source and photodetector. The light source uses frequency division multiplexing modulation method to distinguish two different wavelengths and to shield the interference of ambient light, and also time division multiplexing to drive the light source to obtain higher detection efficiency. After the fNIRS headband was made, it was applied to the interval cognitive test. The results showed that there were interval changes in the concentration of oxygenated hemoglobin （HbO）and hemoglobin （Hb）, which confirmed the feasibility of the fNIRS headband.Subjects were then recruited to participate in a simulated driving experiment, and cognitive ability was measured before and after the simulation. The results of fNIRS imaging showed a decrease in the concentration of HbO in the dorolateral prefrontal cortex (DLPFC) in the late stage of the experiment.Cognitive tasks showed a decline in cognitive ability, indicating that the subjects' brain began to fatigue in the late stage of the experiment, thus verifying the feasibility of fNIRS headband in detecting fatigue.