It is needed to know instantaneous venting volume in venting system design. However, no flowmeter is available to measure instantaneous venting volume in venting system of gas trunk line and calculating formulas now available for instantaneous venting volume are mostly unreliable. Based on Fano equation and basic theory of gas transmission flow, considered is the effect of valve opening on venting time and instantaneous venting amount, applied is the programming language to calculate instantaneous venting amount of venting system in gas pipeline and entered are such parameters as gas pressure, venting line size and valve opening to calculate and obtain the instantaneous venting amount of venting system and relative error between the results obtained by using program calculation and those obtained by SPS software simulation is small. Compared are instantaneous venting amounts and venting pressures in different valve openings and analyzed is effect of valve openings on venting amount, which can provide theoretical guidance for venting system design and field venting.