Depersonalization is a self-aware and unpleasant experience in which the patient has an abnormal sense of unfamiliarity, alienation, or unreality. Depersonalization in a broad sense includes four main aspects: 1. Narrow depersonalization, i.e., feeling that the “I” is detached from the outside world and has become unreal, feeling that the “I” does not seem to exist, which can be described as “no sense of self “2. Disintegration of reality, that is, the patient feels that the external world seems to have changed and become unreal, like a picture or a photograph, without a sense of three-dimensionality and vitality, everything becomes distant and strange; 3. Disintegration of body, that is, the patient feels that the size, weight, hardness and softness of his whole body has changed in a strange way, losing the sense of reality and substance that was normal, seeming to be “4. Emotional disintegration, i.e., feeling that one has lost the ability to experience emotions and cannot love or hate, and feeling painful about it. Personality dissociation is often a result of abnormal changes in the patient’s subjective world. The patient can clearly describe the experience and recognize it as abnormal, but others around him/her do not, and he/she suffers from the inability to be understood. A professional physician can recognize this type of patient’s bizarre experience and can help the patient get early treatment to prevent depersonalization from developing into depression, schizophrenia, and other mental illnesses.