The hydrodynamic parameters of three-vessel system in twin-vessel floatover is studied based on the three dimensional potential flow theory, focusing on the effect of the gap between the transportation vessel and two floatover barges. The analysis results indicate that: the distance between the peaks (or troughs) of the hydrodynamic force coefficient changes regularly with the gap varying, the distance between the adjacent extreme is half wave length or one wave length; when the gap is really small, the hydrodynamic parameters resonate at specific frequency. The recommendation is proposed for the twin-vessel floatover design based on the analysis results.