Abstract: From the perspective of Luhmann's social system theory, law is a normative expectation, adopting a counterfactual attitude of "not learning" so as to reversely achieve the function of time constraints on the whole society. In order to stabilize normative expectations, the functional differentiation of the legal system depends on an organized legal decision system. The decision system promotes the formal establishment of the legal/illegal binary code through the dual patterning of norms, making normative expectations in the legal system obtain a stable realization, thereby overcoming the shortcomings of normative expectations at the general social level. The legal system and the political system thus form a specific structural coupling. The function of law is not to promote social integration or social control, but to ensure that normative expectations are stable in time dimension. The functional model of law does not depend on the sovereign or society, but is given reflexively in the legal system. The law regulates itself at the level of "second-order observation" so as to realize the closure of its operation and the separation of the system.