Studies on the “protest paradigm” have long explored how a society's mass media system frames the social movements occurring within that society. This study adopts a social movement diffusion perspective to sharpen the transborder dimension of the protest paradigm. Specifically, we introduce the concept of the “diffusion-proofing” protest paradigm, given that an understudied and undertheorized function of the protest paradigm is the prevention of the import of exogenous social movements. Taking the Hong Kong Anti-Extradition Law Amendment Bill Movement as an example, we investigate how mainland China's mass media acted to prevent movement diffusion. Four transborder components of the diffusion-proofing protest paradigm are proposed: reactions, attributions, consequences, and solutions. Through content analysis, we argue that by highlighting these transborder components, China's newspapers attributed the movement's rise to foreign intervention, emphasized the movement's negative effects on both Hong Kong and mainland China, and revealed the uncompromising nature of China's reaction and the urgency of cross-border cooperation for containing the movement and its diffusion. Further, we identify two modes of reporting: a mainly descriptive mode that relied on a local framework to depict the movement and its local context and an evaluative mode that emphasized the level of deviance and foreign intervention, thus amplifying the movement's transborder effects and the need to prevent its diffusion. Notably, the level of deviance acts as a mediating channel between the two modes, transforming the local framework into a transborder one. The diffusion-proofing protest paradigm is also found to vary across media types and periods.