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Plasma jet triggered gas gap switch has obvious advantages in fast control switch. The development of the plasma in the ambient medium is the key factor affecting the triggering conduction of the gas switch. However, the plasma jet process and its characteristic parameters are complicated and the existing test methods cannot fully characterize its development laws. In this work, a two-dimensional transient fluid calculation model of the plasma jet process of the gas gap switch is established based on the renormalization-group (RNG) k-ε turbulence equation. The results show that the characteristic parameters and morphological evolution of the plasma jet are basically consistent with the experimental results, which verifies the accuracy of the simulation model calculation. The plasma jet is a long strip with an initial velocity of 1.0 km/s and develops in both axial and radial directions. The jet velocity fluctuates significantly with axial height. As the plasma jet enters the main gap, the pressure inside the trigger cavity drops by 80%, resulting in a rapid drop in the jet velocity. When the plasma jet head interacts with the atmosphere, the two-phase fluid compresses each other, generating a forward-propelled pressure wave. The plasma jet heads flow at high velocity, a negative pressure zone is formed in the middle part of the jet, and the pressure peak decreases gradually with the height. As the value of the inlet pressure increases, the characteristic parameters of the plasma jet increase. The entrainment phenomenon is evident, which leads to an increase in the pressure imbalance of the atmospheric gas medium, leading to significant Coandǎ effect. Compared with air, the characteristic parameters of plasma jet in SF6 are lower, and the morphological evolution is significantly suppressed. The results of this study can provide some insight into the mechanism of action of the switch jet plasma development process.