Two various mathematical approaches of the pneumatic suspension (PS) are proposed and researched to evaluate the reliability and performance of the mathematical models, advantages-disadvantages, and scope of application of each method on the PS of the vehicles. The experimental study of the PS model is also presented under the different harmonic excitations to further reinforce the reliability of the research results. The investigation results indicate that the accuracy of the mathematical approach I (MA-I) determined based on the lumped parameter of the PS model and the mathematical approach II (MA-II) determined via the mass flow rate of the PS is the same results. Besides, the MA-I can easily determine the initial stiffness and damping parameters of the PS and optimize or control the PS, while the MA-II can easily apply for the multi-airbag suspensions using the parallel and long pipes. Additionally, the MA-I should be used for the multi-axles vehicles of the heavy trucks or semi-trailer trucks using the PS that each PS consists of only one air bag connected with a reservoir to improve the ride comfort and road-friendly, conversely, the MA-II should be applied for the two-axle vehicles equipped with the hydraulically interconnected suspensions to enhance the ride comfort and the lateral/roll stability. The research results not only reinforce the reliability and give the advantages-disadvantages of each MA-I and MA-II but also clarify the effectiveness and scope of application of each method on different vehicles.