Depression has always been a hot spot and difficult point of focused research in the field of psychiatry due to its unclear etiology， complicated pathogenic factors， and unclear pathogenesis. Both unipolar depression and bipolar depression are characterized by depressed mood， decreased interest or loss of pleasure， lack of energy or fatigue.Due to the similarity of the two performances， it brings great troubles to clinical diagnosis. This article reviews the differences in clinical characteristics， biochemical metabolism， brain function imaging， facial expression recognition， and cognitive function of unipolar and bipolar depression， providing a theoretical basis for clinical identification of the two， with a view to improving the accuracy and safety of clinical medication.