Functional near-infrared spectroscopic (fNIRS) imaging is an emerging optical brain imaging technique.Numerous studies have been carried out for assessment of mental workload in simple tasks by fNIRS ,but these methods can not be directly applied when the experimental task is complex and integrated.A task decomposition idea is tried to study the variation in cortex blood oxygen with every task feature of complex task using fNIRS,and to find ideas and basis for mental workload assessment in complex tasks.The results show that the activation degree of oxygenated hemoglobin (HbO) in prefrontal cortex (PFC) area makes response to the change of complexity,time pressure,and with or without control strongly;the area of visual cortex (VC) has strong activation degree as the feature of time pressure changes,but weak activation for complexity and with or without control;motor cortex (MC)has strong activation degree with the feature of control existing or not,but its relationship with the change of complexity and time pressure is weak.