Abstract: Misunderstanding of the nature of unlawful infringement is an important reason why judicial practice has too strict control of defense standards. The legitimate defense system is a private remedy in the legal remedy system,and its function is to stop illegal infringements in time. Therefore, illegal infringements should refer to certain and urgent dangerous acts, not including actual harmful acts. The actual harmful behavior theory has had a negative impact on judicial practice and theoretical development, and should be corrected. The danger of unlawful infringement varies in degree, from the danger of the final stage of preparation, the danger of the implementation stage, the danger after completion to the danger of being close to the actual harm result, showing a gradual upward trend. Accordingly, the strength required for defense should also increase as the risk of infringement increases.