As a type of sustainable energy, compressed air energy can be used to drive vehicles, which is applicable to situations requiring explosion prevention and no pollution emissions, including chemical plants and airports. As a key component of an air-driven hydraulic vehicle, an air-driven hydraulic transformer, which is called a hydropneumatic (HP) transformer, is used to pump high-pressure oil for such a vehicle's hydraulic system. To improve the power and efficiency of the HP transformer, in this paper, firstly, a mathematical model of its working process was developed. Secondly, to verify the mathematical model, a dedicated test bench for the HP transformer was established and studied. Through experimental and simulation of the designed HP transformer when the input air pressure, output oil pressure and area ratio are regulated within the ranges of 0.625-0.75 MPa, 1.7-2.2 MPa and 3-5 respectively, it can be concluded that the mathematical model developed in this study is accurate. Furthermore, to improve the output power of the HP transformer, the input air pressure, output oil pressure and area ratio should be increased. Additionally, decreasing the input air pressure from 0.75 MPa to 0.625 MPa may improve the efficiency of the transformer by 14%, increasing the output pressure from 1.7 MPa to 2.2 MPa may improve the efficiency of the transformer by 3%, and decreasing the area ratio of the pistons from 5 to 3 may improve the efficiency of the transformer by 10%. This study can be referred to as the performance and design optimization of air-driven hydraulic HP transformers. (C) 2016 Elsevier Ltd. All rights reserved.