Potential analysis for coupling effect of ship motion and liquid sloshing is considered. A three-dimensional FPSO ship with three partially-filled prismatic tanks are carried out for the global ship response. It is found that the coupling effects are dominant in beam waves. The roll RAOs show typical anti-rolling effect, so that the roll motion decreases dramatically near ship motion resonance and double peaks at sloshing motion resonant frequencies can be observed. The filling mass, that is, the number of filling tank in this work, has significant effect on the ship rolling RAOs, especially at around the ship motion natural frequency.