Abstract: As a method of installing large topsides, floatover installation is time and cost efficient and thereby has been widely used to install offshore platforms. In order to apply the technique to install topsides in the East China Sea, an installation vessel has been developed to adapt to ocean environment conditions in the East China Sea which are mainly with long-period swell. To bypass the restrictions on the motion of the installation vessel in response to the installation process, a slotted stern marine engineering installation vessel and a mooring system are proposed. Considering the combination of long-period swell and short-period wind-sea with wind and current, the maximum motion response of the installation vessel positioned with the mooring system is calculated, and the tension of the mooring lines is verified based on 3D potential theory and coupled time-domain analysis. The results indicate that the long-period swell mainly influences the wave frequency motions of the installation vessel, including heaving oscillation and roll and pitch. The maximum motion response of the installation vessel successfully meets the requirements of installing topside, while the safety of mooring system also satisfies the standard.