As one of the key regions for natural gas hydrate exploration and development in marine sediments in China, low permeable hydrate-bearing sediments in the Qiongdongnan area are classified as clayey-silty deposits which possess complex geological properties such as weak skeletal cementation, abundant micro/nano-scale pore throats, and diverse hydrate occurrence spaces. Therefore, figuring out the dynamic phase transition characteristics of natural gas hydrates in micro/nano confined pore throats and developing quantitative representation technologies are of significance for evaluating hydrate resources and predicting the dynamic production behavior during development. In this work, multi-scale micro/nano fluidic chips were first designed and etched with a minimum pore size of 15 nm based on the microscopic pore structures of hydrate-bearing sediments in Qiongdongnan area. Afterwards, a novel microfluidics-based method for investigating the dynamic phase transition of natural gas hydrates in micro/nano confinements from a microscopic perspective was proposed. The influence of nano-scale pore throats on the dynamic formation, decomposition, and phase transition of natural gas hydrates was revealed. Through characterizing the influence of water activity on hydrate phase transition, a novel representation model for hydrate phase equilibrium considering the capillary effects, surface adsorption, and the effect of soluble salts was developed.  This work provides an effective simulation method and theoretical support for the safe and efficient development of natural gas hydrates in the South China Sea, serving as an important reference for the determination of subsequent development strategies.