Boosters are widely used to obtain high-pressure air. In order to obtain a higher output flow of high-pressure air, a new kind of booster is proposed, namely, the Expansion Energy Used booster (EEU booster). Output flow is improved by utilizing the expansion power of the compressed air to accelerate the piston’s movement. A mathematical model of the booster is constructed and analyzed to obtain the piston’s movement characteristics, variations of air pressure in the boosting and driving chamber as well as the characteristics of the output flow. Evaluation of simulation and experimental flow characteristics shows that the derived model corresponds well to experimental results. For a source and output air pressure of 0.6Mpa and 0.8MPa respectively, the output flow of the booster rises as the Convergent Pressure in the driving chamber increases. At a certain point, the output flow stabilizes and remains at a near constant. For an output pressure of 0.8MPa at the Optimum Work State, the output flow declines with a rise in the boosting ratio. This research lays the foundation for future work on the optimization of the EEU booster.