The purpose of diagnosis is treatment, which is a progressive and in-depth process, and the physician should try to understand the patient as thoroughly as possible; premature diagnosis can limit the physician’s perspective. Once a diagnosis is made, we tend to selectively ignore aspects of the patient that do not fit the diagnosis and accordingly pay excessive attention to features that may confirm our initial diagnosis. A diagnosis of mental illness has the potential to create a sense of inequity in the patient, which can compromise the patient’s ability to express and communicate freely and safely on the one hand, and the practitioner’s ability to establish a collaborative relationship with the patient on the other. Both of these aspects can prevent the patient’s recovery. Rogers, the pioneer of humanistic psychology, believed that everyone has great potential for self-actualization and self-growth, and patients with mental illness are no exception. Only a deep understanding, acceptance and respect for the patient, and the creation of a sincere, equal and safe environment for free communication, can stimulate and reveal the patient’s potential and promote the patient’s recovery. Professor Irvin Yalom of Stanford University, USA, points out that diagnosis can also serve as a self-fulfilling prophetic process. The patient may express symptoms through suggestion and self-referral, and the practitioner’s perception of the patient as a “borderline” or “hysterical” patient and the relationship with him or her may facilitate and promote the patient’s expression of these disorders. In fact, the issue of the influence of physician treatment on the development of a patient’s clinical presentation has long been of interest and concern to clinical psychologists. At the same time, it is also important to remember that our current diagnostic criteria, whether DSM-4 in the United States, ICD-10, or CCMD-3 in China, are not yet perfect and need to be revised almost every few years. The correct diagnosis requires excellent clinical skills of doctors. Once a mental illness is diagnosed, it may require long-term or even lifelong treatment, and the diagnosis may have far-reaching effects on the patient in many ways.