Abstract: The principle of legitimate expectations, promoted by the practice of investment arbitration, is playing an increasingly important role in the interpretation of investment treaties. Some tribunals view this principle as an important element of the FET standard, which will inevitably aggravate the responsibility of the host country under the FET clause. In fact, the principle lacks not only a treaty law basis but also a solid basis of the customary international law and the general principles of law. Only in the general jurisprudence and domestic administrative law can we find some basis for it. The introduction of this principle in the interpretation of investment treaties should be cautiously judged according to the nature of the host government behavior, consider the balance between investor protection and the legitimate regulatory rights of the host country and be applied in a moderate way according to the circumstances. The principle should not only be prevented from evolving into a principle with vague and broad content so that the tribunals can easily expand its interpretation at a low cost but also be prevented from becoming a tool for promoting the "good governance" standard of a few Western countries and damaging the legitimate rights of the host country for regulating foreign capital.