To quantitatively evaluate the wave attenuation performance of oyster shell bags and further investigate the influence of reef roughness parameters on wave energy dissipation, a numerical model of oyster shell bags was established based on OpenFOAM. By changing parameters including relative reef height, relative reef crest width, incident wave steepness, and roughness, the wave attenuation characteristics of oyster reefs under regular waves were analyzed. The results show that the transmission coefficient decreases with the increase in each parameter, among which the relative reef height has the most significant influence. When the relative reef height is 1.0, the influence of roughness on wave attenuation is mainly reflected in the low-value range; when the roughness increases from 0 to 2.5, the transmission coefficient decreases by 23.6%, but as the roughness continues to increase, the transmission coefficient remains almost unchanged. Based on the analytical results, a prediction formula for the transmission coefficient was fitted using regression analysis, which provides a basis for evaluating the wave attenuation capacity of oyster shell bags.