Liquefied natural gas (LNG), as one of the main sources of clean energy, has witnessed great growth in marine transportation in recent years. Due to the potential catastrophic consequences in case of a vessel traffic accident, the guidelines of the design of an LNG Port and the regulation of traffic management require that a mobile safety zone be set up for the transit of an LNG carrier, that is, a moving safety area around the carrier that excludes other ship traffic. To study the impact of a safety zone on channel capacity, this paper has presented a mathematical model to calculate the impact ratio of a large LNG ship on channel capacity considering different speeds and sailing modes. As a case study, an approach channel to a new LNG port that was developed in Yueqing bay, Zhejiang province, East China, has been analyzed during the concept design of the port with the aim of receiving ships with a capacity of 145,000 m3. Based on the model, the impact ratio on the whole channel and the segmented channel when a carrier arrives at and leaves the berth has been calculated. The methodology can support the job of port design and vessel traffic management to improve the capacity, efficiency and safety of a waterway.