Slowly-varying wave drifting force has a crucially important influence on the behavior of the moored medium-sized offshore structures. As to very large floating structures, abbreviated to VLFS, slowly-varying wave drifting force plays a less important role because slowly-varying wave drifting forces on each part of VLFS have different phases and cancel one another. The present paper proposes the phase-difference effect coefficient, which considers cancellation effect of slowly-varying wave drifting force and moment on VLFS. Parametric study is done to have wave drifting moment characteristics with respect to size of VLFS and incident wave angle. Also done is mooring response analysis for 5, 000m-long VLFS, where wave drifting moment is found to play a minor role in contrast with steady wave drifting force.