The seagoing ships using liquefied natural gas (LNG) fuel are becoming popular for transportation ships with the increasing green and low-carbon requirements internationally. The International Maritime Organization has issued the International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels (IGF code), which provides the principal requirements of the loads and design conditions for the design of the fuel tank containment systems using low ignition fuel. However, there is no guidance on the structural strength evaluation of the LNG fuel tank, and the coordination with the loading conditions and evaluation criteria for the transportation ships such as oil tankers, bulk carriers and container carriers, resulting in low operability. It carries out studies by combining the load requirements for the fuel tank design in the IGF code and the relevant requirements in the CSR. A direct calculation method for the structural strength of the fuel tank of an oil tanker using LNG fuel is proposed, including the model range, load conditions, and strength evaluation criteria. The finite element direct calculation for the fuel tank of a CSR oil tank is carried out for full-scale verification. The results show that the method is reasonable and feasible, and it can provide guidance for the structural strength evaluation of LNG fuel tanks of similar ships.