A slotted breakwater is a kind of perforated breakwater with vertical slots in the front wall of the structure. It can provide a sheltered area for loading and unloading of ships and crafts. In this study, a modified model to the one proposed by Suh et al. (2001) was developed to calculate the frequency-averaged reflection coefficient by means of the velocity potential. Using this model, the reflected energy spectrum from slotted breakwaters for normal incident waves can be calculated. A comparison with experimental and analytical works shows that this model can accurately predict frequency-averaged reflection from this type of breakwater. This model can provide a good trend to predict frequency-averaged reflection and is a powerful tool for predicting the reflected energy spectrum from slotted breakwaters. The effect of effective parameters such as relative wave chamber width, relative significant wave height, significant wave period, and porosity of the slotted wall was also considered on the frequency-averaged reflection and reflected energy spectrum in this model. One of the most important results is the calculation of frequencies in the reflected spectrum that have a minimum energy close to zero, denoted as zeroth frequency points. This means that a large part of the wave energy in such frequencies is absorbed by the structure of the breakwater.