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 on VLFS. The proposed method is validated with the comparison to field measurement for a 1, 000m-long VLFS. It is also shown that an assumption of perfect reflection of incident waves at VLFS' edges is valid and advantageous in designing the mooring system of VLFS.