A large development at the coastal water causes serious changes to the wave energy transmission. Especially, variation to the wave field introduced into the inner harbor area induces strong impact to wave energy transmission system. In order to reduce damages of ships and harbor structures due to wave attack, it is necessary to predict the change of wave field. However, harbor tranquility review is being conducted on the wind wave and swell of 10∼20sec periods to be an engineering problem. The long period waves more than 20sec still show not just damages to the vessels and harbor structure in real field, but also give difficulties on port operation by lowering the annual berthing ratio. This study deals the extended mild-slope equation model simulation for predicting wave responses at Sadong port, Ulundo with a new airport plan, Korea. Possibilities of harbor oscillation reduction introducing a resonance basin are discussed. For the existing configuration of Sadong port, the resonance basin and breakwater gap give larger reduction to amplification factors appeared, such as 20% reduction for Case 1 and 40% for Case 2, at the particular resonance modes. For the analysis of berthing capability at the new port, a 500m extension of breakwater was added with an existing 900m north breakwater and compared with the present planform of port. Due to addition of the extended breakwater at north side, the unexpected variation of harbor responses in the new port show up to a noticeable level. In this case the variation occurred in a way of wave height reduction to the harbor basin but induces difficulties to both the ship passage and pier face. Thus, in this simulation, we have dealt with the response characteristics of long period waves on the water area expansion as per the port development plan, together with introduction of a resonance basin for control of oscillation wave periods, using the offshore field wave data collected. Based on those analyzed results, it is to be recommended to the future port rearrangement or reengineering.