Abstract: In the design and stability analysis of marine engineering projects such as embankments, offshore wind turbines, submarine pipelines, offshore oil platforms, and breakwaters, the initial static shear stress state of the seabed foundation soils is an essential factor that must be considered. A series of constant-volume cyclic direct simple shear tests are conducted on soft clays from the Yangtze Estuary and the Cangnan offshore region to systematically investigate the cyclic failure modes and cyclic resistance characteristics of marine clays under combined static and dynamic stress conditions. The results indicate that, based on the relationship between the initial static shear stress ratio (ISSR) and cyclic stress ratio (CSR) as well as the failure modes of the soils, the test results can be categorized into four types: Category Ⅰ: ISSR = 0, with the failure mode being cyclic mobility failure; Category Ⅱ: 0 < ISSR < CSR, with the failure mode being cyclic mobility failure; Category Ⅲ: 0 < ISSR < CSR, with the failure mode being accumulated plastic strain failure; Category Ⅳ: 0 < CSR < ISSR, with the failure mode being accumulated plastic strain failure. Based on fluid mechanics theory, a quantitative evaluation method for the failure modes of marine clays under combined static and dynamic stresses is established, revealing a negative correlation between the stress reversal coefficient χ and the failure apparent viscosity ratio η_f/η₁, where threshold stress reversal coefficient χ_th and threshold failure apparent viscosity ratio (η_f/η₁)_th can serve as key indicators for distinguishing cyclic failure mode. As ISSR increases, the cyclic resistance of marine clays gradually decreases. A new cyclic resistance evaluation method is established that accounts for the influence of ISSR and is applicable to different failure criteria. Additionally, an empirical relationship between the sensitivity coefficient ξ and plasticity index I_P approximately in the range of 15 < I_P < 30 is proposed based on external data from the literature.