A mathematical model for a closed cylinder with a holed piston was established to investigate the operational characteristics of a gas spring. An ideal gas working fluid and an adiabatic process were assumed in the model. Simulation and experimental results for a typical design and operational condition showed good agreement. The model was then used to investigate the effects of the orifice diameter, the filling pressure, and the velocity of the piston on the applied force and operating pressures of two chambers in the gas spring. The results showed that the orifice diameter and the piston velocity had significant effects on the pressure difference between the two chambers. A 0.05-mm reduction in the orifice diameter led to an approximately two-fold increase in the pressure difference. A 10% increase in the piston velocity resulted in an approximately 25% increase in the pressure difference. The orifice diameter and the piston velocity had a greater effect on the chamber with lower pressure than on the chamber with higher pressure. The force applied to the piston also varied with the piston velocity, the orifice diameter, and the filling pressure due to irreversibilities including the throttling process through the orifice and friction between the wall and the piston. The present model and results are expected to supply useful information about a new gas spring design.