The present study aims to investigate the effect of liquid sloshing of inner tanks on the dynamic responses of an underwater soft yoke mooring system. For this purpose, inner tanks with different filling levels were examined under different external wave loads in towing tank experiments. The specified waves generated by a piston-type wavemaker are validated. It is found that the dynamic response of the new underwater soft yoke mooring system is sensitive to the inner tank sloshing and the external wave load type and the effect become more significant when the wave becomes more turbulent. Besides, the filling depth of the inner tank plays a vital role on the dynamic response of the mooring system. Particularly, a significant increase in the force acting on the mooring legs was found when the floating structure was configured with two inner tanks.