With the popularization of AIGC technology and the explosive growth of the short-video industry, secondary creation in short videos has become a core issue involving both cultural communication value and copyright disputes. Its creative forms have evolved from traditional editing and montage to diversified modes such as AI “remix” and template-based generation, leading to blurred copyright boundaries and difficulties in infringement determination. Based on the current framework of the Copyright Law and judicial practice cases, this paper focuses on the core controversies over copyright boundaries of secondary creation in short videos, and systematically analyzes key issues including the identification of fair use, the localized application of transformative use, and the division of platform responsibilities. It also sorts out the divergences in the judicial application of the “three-step test” and the “four-factor test”. Finally, it proposes an infringement determination path and systematic improvement suggestions featuring “typed definition— hierarchical determination— collaborative governance”, so as to provide legal support for balancing original incentive and creative freedom and regulating the development of the industry.