Abstract: In petrochemical field, the pressure vessels are widely applied for storing and transporting compressed natural gas. In order to guarantee the safety of equipment and pipeline, the gas filled in the pressure vessels should be released under some special conditions. Based on the first law of thermodynamic, mass balance and AGA8 equation of state, a novel dynamic model was established to stimulate the relief process in this paper. According to this model, the variation of the compressor parameters, such as pressure, temperature, compressor factor and adiabatic index were predicted. Meanwhile, the influences of external temperature, gas component and orifice area on discharge time were investigated. Besides, a method for calculating orifice diameter was presented. The simulation results showed that the compressor factor and adiabatic index are variables and the gas component has significant effect on discharge time, which should not be ignored in calculations. Moreover, the discharge time increased with the elevation of the external temperature. All the above findings could provide reference and support for the design and optimization of the pressure vessel vent system.