The surface of metal systems exposed to ionizing radiation (X-ray and γ-ray) will emit high energy electrons through the photoelectric effect and other processes. The transient electromagnetic field generated by the high-speed electron flow is called SGEMP, which is difficult to be effectively shielded. An ongoing effort has been made to investigate the SGEMP response in vacuum by numerical simulation. However, the systems are usually operated in a gaseous environment. The objective of this paper is to investigate the effect of low-pressure air on the SGEMP. A three-dimensional hybrid simulation model is developed to calculate the characteristics of the electron beam induced air plasma and its interaction with the electromagnetic field. In the hybrid model, the high energy photoelectrons are modelled as macroparticles, and secondary electrons are treated as fluid for a balance between efficiency and accuracy. A cylindrical cavity with an inner diameter of 100 mm and a length of 50 mm is used. The photoelectrons are emitted from one end of the cavity and are assumed to be monoenergetic (20 keV). The photoelectron pulse follows a sine-squared distribution with a peak current density of 10 A/cm2, and its full width at half maximum is 2 ns. The results show that the number density of the secondary electrons near the photoelectron emission surface and its axial gradient increase upon increasing air pressures. The electron number density in the middle of the cavity shows a peak value at 20 Torr. The electron temperature decreases monotonously with the increase in pressure. The low-pressure air plasma in the cavity prevents the generation of the space charge layer. The peak value of the electric field is an order of magnitude lower than that in vacuum, and the pulse width is also significantly reduced. The emission characteristic of the photoelectrons determines the peak value of the current response. The current reaching the end of the cavity surface increases at first and then decreases upon increasing pressure. The plasma return current would suppress the rising rate of the total current and extend the duration of current responses. Finally, to validate the established hybrid simulation model, the calculated magnetic field is compared with that of the benchmark experiments. This paper helps to gain a better understanding of the electron beam induced air plasma at low pressures and achieve a better prediction of the SGEMP response in a gaseous environment. Compared with the particle in cell - Monte Carlo collision method, the hybrid model adopted can greatly reduce the computational cost.