Abstract: Gassy soils have massive gas bubbles dispersed in the pore water, and this type of special structures leads to complex mechanical properties which are inherently controlled by the total stress and the pore pressure. However, mechanical properties of gassy soils when subjected to high effective stresses remain elusive. In this study, intact marine sediments were cored from an offshore region rich in shallow gas near the city of Zhoushan, and the sediments, after indoor testing for basic physical and chemical indexes, were remolded to prepare gassy soil specimens by using the zeolite method. Loading and unloading were performed twice on the gassy soil specimens under laterally confined condition, and effects of the gas content on deformation behaviors as well as seepage characteristics during loading and unloading were unveiled. It is shown that coefficient of compressibility of marine gassy soils decreases during loading, and the compression index together with the expansion index increases with increasing gas content. Pore gas and water are produced mainly during the virgin loading, and more than one half of the cumulative volumes of produced gas and water are squeezed out of gassy soils at the beginning of the virgin loading. The hydraulic conductivity of water saturated soils monotonously decreases when the soils subjected loading. However, the hydraulic conductivity of marine gassy soils firstly decreases, then changes to increase, and finally returns to decrease during the virgin loading.