Offshore culture becomes the most attractive industry in last decades because of overfishing and excessively pumping groundwater causing land subsidence social problems. Oyster culture is one of offshore culture with several advantages such as easy set-up and without feeding cost. In this study, we focus on a longline-style oyster aquaculture system because it is recyclable, high growing efficiency and advanced technique against traditional tidal flat culture. This study establishes a numerical model through a lumped mass method, then employs fourth order Runge-Kutta method to solve the system of motion equations and evaluates the tension on the anchor rope. The results show the discrepancy between the numerical model and experimental data is lower than 4% in current-only situation, and similarly in wave-only situation the discrepancy is lower than 6%. The comparison results indicate that the numerical model is capable to predict the environmental loadings on longline-style oyster culture system. An in-situ case study of a longline-style oyster aquaculture system via regular and random waves, located in the Penghu Bay, is investigated based on the sea-state, Chebi Typhoon occurred in June, 2001, causing a catastrophic damage to the cage aquaculture. The conclusions of dynamic responses of the case study may be utilized as guidance for the local oyster farmers to build and protect their oyster culture system in the Bay.