Restriction orifice is more and more widely applied in pressure relief and vent system under fire conditions at gas gathering station. There are various methods for orifice diameter calculation and calculation results differ from one another. Conducted is contrastive analysis on three kinds of common orifice diameter calculation methods and described are characteristics and application scope of each method. Taking the gas station of Turkmenistan Amu Darya gas field as an example, calculated are restriction orifice diameters in pressure relief and vent system under fire conditions at the gas gathering station and it is recommended that combination of two calculation methods together may be more efficient.