Abstract: The emergency blowdown system provided safety guarantee for the pipes and equipments in the gas and oil gathering station. Especially under fire case, the pressurised medium must be relieved quickly to avoid the equipment rupture or explosion in consequence of the decreased material strength at high temperature. The blowdown rate is mainly influenced by the size of Restriction Orifice (RO), of which the calculation is worth deeply studying to meet the relief requirement under extreme conditions. In this paper, based on the current restriction orifice sizing method and the feature of fire cases, the adaptability of the current equation on fire case is analyzed, and the sizing method for fire induced relief is then optimized by using HYSYS dynamic simulation model. The results showed that the evaporation of light condensates and vapor thermal expansion under fire case could not be thoroughly taken into consideration with the current method. Thus, the determined orifice size from the current method could not meet the emergency blowdown requirements due to the huge evaporation rates of the light condensates after heated. Instead, the HYSYS dynamic simulation method could present more realistic simulation and its calculation results were more reasonable than the conventional methods. Overall, this study provided reference for restriction orifice sizing in emergency case.