The principle of inverse mask is to identify the change of facial expression through the analysis of human face, so as to infer the real emotional state of people. This principle is based on the relationship between human facial expressions and emotions. By analyzing the micro-expressions, eyes, lips and other details of human faces, we can infer the real emotional state of people, including anger, disgust, fear, happiness, sadness and so on. The principle of reverse face instrument is widely used in psychology, social engineering, intelligence gathering and other fields.