Article 153 of the Civil Code stipulates the application rules of mandatory norms and public order and good customs clauses, which are typical of the application of the ex ante and the ex post models that combine restrictions on freedom of contract and should be interpreted systematically. To coordinate the application of the two models in determining the validity of contracts, the scope of application of the ex ante model with mandatory norms as the core is reasonably defined, and the boundary of the application of mandatory norms is clearly defined. Norms that are not explicitly stated by the legislator should be based on the application of arbitrary-norm presumption principle; on the premise of respecting the autonomy of the parties, the court should play the complementary role of the ex post model according to the principle of public order and good customs, with a view to making a clear determination of contract validity.