The first part mainly discusses the concept of legality of investigation strategy. This part starts with the concept of investigation strategy, analyzes the basic characteristics of investigation strategy and defines the connotation of investigation strategy. On this basis, the investigation strategy and investigation measures are distinguished. With regard to legitimacy, this paper starts with the question of "what is law", defines the concept of legitimacy in this paper on the basis of drawing lessons from the concepts of politics, law, philosophy of law and other disciplines, distinguishes legitimacy from legitimacy, and further analyzes the content of legitimacy of investigation strategies, that is, the evaluation criteria of legitimacy, including philosophical evaluation and legal evaluation. The second part studies the theoretical boundary of the legitimacy of investigation strategy. Due to the lack of direct regulation of investigation strategy at the legal level, the theoretical research on the legality of investigation strategy should not be limited to the legal level, but should have a broader vision and draw theoretical support that not only conforms to the basic spirit and principles of the law, but also effectively promotes investigation. In the aspect of legal boundary, the design and application of investigation strategy need to meet the legitimacy of subject and means. The legality of means mainly includes prohibiting the use of strategies that may lead to the acquisition of false evidence, prohibiting the implementation of investigation strategies that violate the Constitution and laws, prohibiting investigation strategies that infringe on the relative person's right to freedom of will and controlling investigation strategies. Judging from the scientific evaluation standard, the investigation strategy should be effective and economical. In addition, the standard of legality of investigation strategy has moral boundary. The third part studies the practical application of the legality of investigation strategy. According to different standards, investigation strategies can be divided into different types. Threatening, luring and cheating strategies, which are clearly stipulated in the Criminal Procedure Law but widely used in practice, all have their own standards when grasping their legitimacy. Theoretically, investigation strategies mainly include questioning witnesses, interrogating criminal suspects, seduction investigation, controlled delivery and secret investigation.