Abstract: Objectives the application of various free-flooding ship is increasing year by year and the characteristic of this type of ship is the presence of both tank sloshing and connectivity between internal and external seawater. In order to study this specific phenomenon numerical simulation is carried out. MethodsOn a CFD commercial software platform, a 3D free-flooding ship section with holes on both port and starboard sides model is established and the forced roll motions under different given roll periods and amplitudes are simulated in a viscous numerical water tank. Then the relationship between the mass flow rate of the water flow at the openings and the applied working conditions is studied. Results The results show that if the rolling period is bigger, the flow amplitude ratio will also be bigger and the time difference between mass flow and ship rolling velocity will be smaller. ConclusionsThe method adopted can accurately simulate the water exchange and this water exchange will exhibit strong regularity. And based on this, further study can be made on the hydrodynamic performance of free-flooding ship.