Models that simulate the signature of single airguns form the basis for modeling the signals of airgun arrays. Most of the existing models assume that the air inside the produced bubble is ideal gas, which may lead to errors because of the high operating pressure of the airguns. In this study, we propose a model that precisely simulates the signals of single airguns by applying the Van der Waals equation based on the Ziolkowski algorithm. We also consider a thermodynamically open quasistatic system, the heat transition between water and 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) the reflection coefficient of the sea surface under the actual state and depth of the source affects the notch caused by the surface reflection, (3) the computed signature with the proposed model is very close to the actual data, and (4) the proposed model accurately simulates the signature of single airguns.