Mental illness is occasionally dismissed as merely a psychological problem, with the underlying cause of brain pathology often ignored. This lack of knowledge has hindered the development of psychiatric diagnosis and related pathophysiology research. Psychiatric medicine has also failed to adopt objective diagnostic criteria such as those employed in internal medicine. Accumulating evidence reveals that mental illnesses may have underlying neurological causes. Structural and functional magnetic resonance imaging can enable the investigation of novel biomarkers for assessing psychiatric disorders. Additionally, the emergence of machine learning techniques has resulted in considerable breakthroughs and far-reaching consequences in the analysis of brain structure and functional data. Artificial intelligence and brain imaging big data may also provide an opportunity for the establishment of a scientific method for objectively assessing the diagnosis and severity of mental illness symptoms. This can effectively help clinicians in assessing mental illnesses and improve the quality of mental health care. In particular, brain-based psychiatric diagnosis may further delineate the pathological mechanisms of psychiatric symptoms, promote the understanding of the human brain (the most complex organ of the human body), and accelerate the re-evolution of modern psychiatry