The suspension system for a trailer is disclosed. The suspension system includes a torsion axle and an air spring operating to accommodate a wide range of a load weight applied to the trailer without any change in ride comfort. The air spring can be adjusted for its pressure inside to cover a heavy load weight, while the torsion axle can have relatively softer spring materials therein to cover a light (and zero) load weight. The air spring and the torsion axle are arranged in series to provide adequate suspension based on the load weight.