Taken a project in a block in Bohai Sea as an example, the pressure-holding sampling method is used to obtain the seabed soil samples to be assessed, and the gas components are determined to qualitatively determine the origin of the shallow gas. The excess pore water pressure in the formation is obtained by dissipation of pore pressure test based on cone penetration test to quantitatively evaluate the reduction effect on the bearing capacity of the formation with shallow gas. The results show that the main components of the shallow gas in the study area are methane and carbon dioxide, and the shallow gas content is relatively low. The shallow gas in the inland layer is possibly biogenic gas. The ratio of the excess pore water pressure to the overlying effective stress in the formation with shallow gas is about 3%, and the bearing capacity reduction factor in the formation with shallow gas is from 1.0% to 1.1%. The method adopted can effectively evaluate the impact of the shallow gas on offshore infrastructure and production safety, and is of promotion value.