The increased density in a liquefied natural gas (LNG) tank caused by boil‐off gas (BOG) is a key factor to consider for LNG‐fueled ships with Otto cycle engines. This study conducted dynamic simulation using Aspen HYSYS to solve LNG aging in the fuel gas supply system (FGSS) of an extra‐long stroke dual fuel (X‐DF) engine. The aging phenomenon is severe in the Otto cycle internal combustion engine because heavy hydrocarbon (HHC) components must be removed during the process to satisfy the methane number (MN) requirement. HHC components such as ethane, propane, butane are separated in the HHC removal process so that HHC can be returned to the LNG storage or fuel tank without increased risk of equipment failure relative to steady‐state design conditions. Dynamic process simulations for four cases were conducted to verify the behavior of LNG aging for two fuel compositions and two fuel tank capacities. The results indicate that the different composition of LNG affects the pump duty, the duty of heat exchanger and BOG compressor in steady‐state simulation. Additionally, based on the analysis results, solutions to the problem caused by LNG aging for the FGSS in an X‐DF engine were provided.