The Moon Jae-in government has put forward regulatory reform of newly emerging technologies based on the comprehensively negative principle. This regulatory principle was achieved through the methods of flexible regulatory rulemaking and regulatory sandbox. However, unlike regulatory sandbox, which has been actively developing, the reform regarding flexible regulatory rulemaking displayed slow progress. The purpose of this article is to present flexible regulation as a new principle for regulatory reform of newly emerging technologies by defining its concept and rationale theoretically. Two tasks are tackled. First, the article analyzes regulatory reform cases in the fields of smart mobility, bio-health and fintech and points out limits of flexible regulatory rulemaking as a tool for making regulation flexible. Second, it defines flexible regulation as diversifying the choices of regulatees that achieve the regulatory objectives across regulatory governance, which comprises rule structure, enforcement structure and regulatory feedback. Here, the flexibility of regulatory rulemaking becomes a way of achieving flexible regulation, and the principle of comprehensively negative becomes one way to achieve flexible rule structure. Drawing on regulatory theories that have been presented as alternatives to command-and-control regulation, this article builds systematic ways to increase the choices of regulatees, establishes the system of classifying flexible regulation and presents policy implications and suggestions for future research.