The Chinese national legislature has enacted a national security law (NSL) for Hong Kong. This paper applies the theory of national security constitution as developed by Professor Koh to examine the impact of the NSL upon four core constitutional principles in Hong Kong’s mini-constitution which underly its national security constitution and to evaluate the nature of the impact. The paper argues that the NSL has, instead of applying and supplementing the existing underlying constitutional principles in the mini-constitution, changed them to various degrees, and the pre-NSL bifurcated national security system for Hong Kong and mainland China has been replaced by an integrated national security system under the NSL. The impact caused by the NSL is so significant as to amount to a permanent paradigm shift to a new post-NSL national security constitution. The paper also argues that the theory of national security constitution has its limitation in its application to subnational Hong Kong because its mini-constitution and the underlying principles therein can be modified by national legislation, such as the NSL, of its sovereign, China. Through a comparative study with the USA, the paper proposes that the theory of national security constitution needs to be modified by adding that different effects may occur to a sub-national national security constitution depending on the source of the framework national security legislation. Hong Kong’s failure in its constitutional duty to enact national security legislation under Article 23 of the Basic Law has led to the enactment of the NSL by China. Such legislation from the sovereign has changed the underlying constitutional principles and is fundamentally different from sub-national framework legislation that only implements and supplements those principles. However, a comparative study with Macau indicates that the theory of national security constitution is still applicable to a sub-national entity such as Macau so long as China as sovereign exercises self-restraint and any framework national security legislation is enacted at the sub-national level.