Abstract: To ensure the safety of offshore floating wind turbine system,a novel the liability assessment method based on comprehensive multi-failure modes is proposed in this paper. The OC3 floating spar（Hywind）with 5 MW baseline wind turbine developed by American National Renewable Energy Laboratory（NREL）is applied as the target model. The coupled dynamic responses of OC3-Hywind are simulated under various sea states. And the reliabilities of several critical members under both ultimate strength failure and fatigue failure modes are calculated respectively. Then,the multi-mode reliability assessment is applied to calculate the structural reliability. The results demonstrate that the failure probability of the offshore floating wind turbine by the multi-mode reliability assessment is greater than that of any single mode. It is more efficient and accurate to estimate the structural reliability by this method.