The interpersonal meaning of American policies on US-China trade can be explored by comparison of the characteristic Modality System in the foreign trade policies of Biden and Trump administration from a self-constructed corpora within the framework of Appraisal Theory. The findings are as follows: 1) Compared with the trade policies of Trump administration, the frequency of Modality System in Biden's policies is higher especially with more Deontic Modality, whereas the distribution of Epistemic Modality and Volitional Modality is not significantly different; in terms of the value of modality, more modalities of high-value and fewer that of medium-value and low-value are used in Biden's trade policies; 2) different types of Modality are used for judgment, different values are used to regulate the force of discourses, and the pairing of evaluative lexical resources with modal words shifts the source of pressure and achieves the expansion and compression of the dialogue. Thus, the Modality System has the interpersonal meaning of expressing different degrees of the subject's attitude and position, shifting the responsibility of modality, and expanding and compressing the dialogue space.