Abstract: Impairing quality assurance function is similar to impairing source identification function. Both of them can hinder the trademark’s role in conveying information. Impairing quality assurance function can constitute an independent reason for determining trademark infringement. In new types of trademark infringement cases that do not involve likelihood of confusion, courts have already treated “damaging quality assurance function” as an independent reason for determining trademark infringement. However, the essential connotation of quality assurance function remains ambiguous, and the criteria for its determination lack clarity, which increases judicial uncertainty. Clarifying the connotation of quality assurance function and defining the methods for determining infringement and defenses against infringement can help reduce this kind of uncertainty. In terms of connotation, quality assurance represents the stability of product quality. Regarding methods for determining infringement, the “substantial change” criterion can be beneficial but still vague. The core judgment criterion should be based on consumer expectations, combining the tripartite classification of core characteristics, formal characteristics, and extended characteristics of quality in the field of commodity science, and applying the expectation versus performance comparison method for judgment. In terms of infringement defenses, full disclosure can serve as a defense against damaging quality assurance function. The sufficiency of disclosure should be comprehensively assessed based on factors including but not limited to the manner of disclosure and the content disclosed.