Abstract: The digital public library is supposed to provide a path for citizens to exercise their rights to education and cultural development as entrusted by the constitution, but the revision of relevant laws did not make them free from frequent infringement lawsuits, and the ultimate interest flow of its operating mechanism mostly points to the database provider. Existing theoretical bailouts and practical solutions all have a strong functionalist stance, and lack effective methodology as an argument support. Currently, libraries are still facing “infringement accusations + digital transformation + one-way public welfare + social tasks” – a quadruple limitation. The principle of proportionality has dual meanings of ontology and methodology, and can be used for copyright law and copyright restrictions at the same time. Therefore, the principle of proportionality can be used as the overarching principle, with fair use as the core principle, supplemented by technical measures, setting of blank window periods, punitive damages, and other institutional construction, so that a three-stage restriction layout with two-way public welfare as the requirement can be built for the digital public library to strike a balance between private rights restriction, rights protection and social education.