Models that simulate the signature of single airguns form the basis for modelingthe signals of airgun arrays. Most of the existing models assume that the air inside theproduced bubble is ideal gas, which may lead to errors because of the high operating pressureof the airguns. In this study, we propose a model that precisely simulates the signals of singleairguns by applying the Van der Waals equation based on the Ziolkowski algorithm. We alsoconsider a thermodynamically open quasistatic system, the heat transition between waterand gas, the throttling effect of the port and the bubble rise, and the effect of the sea surface.Modeling experiments show that (1) the energy of the source increases and the signal-to-noise ratio of the signature wavelet decreases with increasing seawater temperature, (2) thereflection coefficient of the sea surface under the actual state and depth of the source affectsthe notch caused by the surface reflection, (3) the computed signature with the proposedmodel is very close to the actual data, and (4) the proposed model accurately simulates thesignature of sinale air~uns.