Attention deficit hyperactivity disorder (ADHD) is a common psychological and behavioral disorder in children, adolescents, and even some adults. Several symptoms are observed in ADHD patients, such as difficulty in paying attention, hyperactivity, impulsivity, and cognitive abnormalities. In the past, clinical diagnosis of ADHD was mainly conducted through a paper test scale based on relevant standards. Subsequently, although the time efficiency of test based on computer has improved, disadvantages have remained, for those tests being too tedious to attract the interest of individuals and being restricted by the content and the type. In this study, a combination of virtual reality, eye tracking, and electroencephalogram (EEG) signal acquisition technologies was used to conduct diagnostic assessment and testing on patients with ADHD. Selective attention, sustained attention, abstract reasoning, and cognitive transfer abilities were evaluated by performing visual and auditory continuous performance test (CPT) and the Wisconsin card sorting test (WCST) in a 3D virtual classroom with optional distraction factors. This study also preliminarily analyzed the eye tracking and EEG data and validated their effectiveness and convenience in ADHD diagnosis. The system may provide a deeper level of ADHD diagnosis and cognitive rehabilitation.