Floating offshore wind turbines have the potential to commercially convert the vast wind resource in deep-water area. Compared with fixed-bottom wind turbines, motions of the floating foundation complicate vibrations and loads of the wind turbine in offshore environment. To alleviate the responses of the wind turbine, this study investigates the use of fore-aft tuned mass damper (TMD) in nacelle/tower for passive control of a semi-submersible offshore wind turbine. A simplified structural model, considering the degree-of-freedom of platform pitch and surge, tower tilt and TMD translation, is proposed in the light of motion features of semi-submersible platform. After identifying ten unknown parameters, the correctness of the deterministic model is validated by pitch free decay responses. The mass, stiffness and damping of TMD are optimized using both method of exhaustion and genetic algorithm to avoid local minimum. Six optimized TMD devices are evaluated under three kinds of realistic environment conditions. The control effectiveness is assessed by the extreme and fatigue response reduction ratios. It is found that the high stiffness TMDs that directly dissipate the energy of tower oscillation exhibit an overall stable performance. Similar to the spar-type foundation, the TMDs in the nacelle/tower are capable of extending the service life of floating wind turbines.barge, spar and tension lag platform types of floating wind turbines, respectively, and optimized the tuned mass damper (TMD) configurations for each type of foundation using a genetic algorithm. They also concluded that the active control device achieved more load reductions than passive TMD. Compared with the other floating foundation, the barge-type floating wind turbines exhibit larger response [8]. Taking the ITI energy barge 5 MW FOWT as an example, He et al. [9] pointed out that the maximum suppression rates of vibration responses vary from 20% to 50% as the mass ratio changes from 0.5% to 2%. Furthermore, Li et al. [10] investigated the active control strategies of TMD device and found that the wind turbine fatigue loads and generator error could be reduced effectively. With regard to the spar-type wind turbine, Si et al. [11,12] established a 5-DOF dynamic model for the Offshore Code Comparison Collaboration (OC3) Hywind spar-type wind turbine, and studied the effectiveness of TMD devices installed in nacelle and platform respectively. also investigated the effectiveness and feasibility of single and multiple TMD devices on a spar-type FOWT, and claimed that the spar TMD is more effective than the nacelle TMD.Apparently, TMD devices are effective in vibration and load mitigation for both barge-type and spar-type FOWTs. However, to the authors' knowledge, no studies have been reported to concern this issue of the semi-submersible foundation FOWTs. Nevertheless, currently, the semi-submersible foundation occupies the largest number of the few installed full-scale FOWT prototypes for its conspicuous capability in supporting the operational condi...