In order to find out whether the loading medium has an effect on the mechanical properties of geomembrane bulging deformation, HDPE geomembranes with three thicknesses of 0.2 mm, 0.3 mm and 0.6 mm were selected as the research objects. Using the special test equipment for bulging deformation, we used air and liquid as the loading media respectively. At the injection rate of 1 kPa/s, the spherical bulging deformation test under annular constraints was carried out, and the microscopic morphology changes and differential failure mechanisms of geomembrane bulging failure were emphatically analyzed by means of a polarizing microscope. The results show that under the same environmental conditions, the bursting pressure under liquid bulging is about 34% higher than that under air bulging, and the bulging height under liquid bulging is about 22% higher than that under air bulging. The bulging deformation of geomembranes under air and liquid bulging shows different stages of elastic, yield, strengthening and failure deformation. The geomembrane presents a linear failure surface during air bulging, which is manifested as uniform tensile deformation and fracture of the polymer chain segments at the micro level. The geomembrane presents a banded failure surface during liquid bulging, which is manifested as uneven tensile deformation and fracture of the polymer chain segments at the micro level. The influence of the loading medium on the mechanical properties and failure characteristics of the spherical bulging deformation of the geomembrane is mainly related to the properties of the medium itself, the distribution of pressure load under the membrane and the reorientation and recrystallization of polymer chains. According to the research results, it is recommended to use liquid medium to carry out the strength test for the bulging failure of the geomembrane.