Based on batch settling tests, a model describing the compression settling process of activated sludge was developed and validated by experiments. Furthermore, a theoretical equation for determining the critical point when the compression settling stage commences, and a new velocity function for the compression settling were deduced from the model. By combining the new model and the conservation of mass, it was proved that the Vesilind function was also capable of describing the compression settling velocity on condition that the appropriate parameters were estimated. Dividing the complete settling process of activated sludge into the zone settling and compression settling stages, and describing them by the Vesilind function with different parameter sets was more reasonable for characterizing the complete settling process of activated sludge. The method was applied to predict the sludge blanket height during batch settling tests, and the results showed that the settling processes could be simulated well.